Wow. Just spent an hour typing out a lengthy reply about one of my favorite powerset combinations and got "the form you have submitted is no longer valid" and lost the post. I wanted to cry. This version will be a bit briefer since I want to sleep today.
Is this character already 50, or is it something that you're working on? Do you plan to play on a specific size group (solo, duo, 8-man SFs?) or making more of a generalist build? Do you have a special goal in mind with your set bonuses, or getting whatever you can squeeze in?
To me, it seems that you've sacrificed some of the build's uniqueness and functionality for the sake of set bonuses, but I can't tell what you had in mind when trying to get them. Personally I don't feel the need to go out of my way for global recharge, but I do love ranged defense.
Some specific changes I'd make:
Twilight grasp: I'd drop the 5 and 6 piece bonus for 2 heal IOs. It's a very powerful heal (arguably the most powerful in the game) for when things do get through your massive to-hit debuffs.
Frost breath: I can't imagine not having this power on teams. I can usually squeeze in 2-3 per spawn. It's no fire breath, but it's very efficient if hitting multiple targets. Ice is fairly mediocre at AoE already; no need to give up half your AoE arsenal. Different people have different opinions about it, but I'd advise at least trying it out if you haven't already.
Howling twilight: I'd steal the slots from this and put them elsewhere. Great power. It works fine for me with 1 recharge though. (or add a slot and put in the stun purple set maybe) Edit to add: It doesn't work too well as a control power anymore, in my opinion. Mag 2 stun and mag 3 "afraid" just creates chaos. Minions are stunned for 10-15 seconds, which is fairly pointless. Lieutenants run to the other side of the map before walking back slowly and giving each other pep talks, which is fairly annoying unless you need to rez people.
Soul drain: I'd drop this entirely. I found it unwieldy to use for too little benefit, and I wanted to use its slots elsewhere. I took power sink instead (which can be used, with the help of any blue, to basically negate the crash from blizzard).
Dark servant: I'd add heal IOs to it. He takes care of minor "maintainence" healing in the ranged group for me most of the time while I patch up anyone who suddenly loses half their health.
Single target blasts: I'd 6 slot thunderstrike here. 11.25% ranged defense and 21% accuracy from 3 sets.
Freeze Ray and Petrifying Gaze: Having these two and fearsome stare was a large reason that I rolled this combination of powersets. I'd advise taking them both (probably late in the build though, due to the way mez lasts longer at higher level) and 4-6 slotting. Baslisk's gaze (4) and lockdown (6) are both good sets to throw in these. I wish bitter freeze ray was better/faster and I'd consider taking that too.
Blizzard: I'd use positron's blast (5) and a recharge here. Or drop it to 4 slots (2dmg/2rchg) if you need to move 2 elsewhere.
Shadow fall: With no endurance reducers, this one toggle is going to eat about 20% of your endurance regen. Nomnomnom 
Add in dark embrace plus not having power sink, and I forsee some endurance problems.
To answer your specific questions:
1) Hasten isn't overkill in my opinion. It's a very powerful return for a small investment. I don't think it's really mandatory with this build, but I would advise keeping it -- particularly if you drop some of the recharge bonuses, or if you're still working towards them.
2) You don't really need more defense, but out of the available set bonuses, I feel that it's one of the best to aim for. With high ranged defense it's a lot less scary to take the pitiful alpha after fearsome stare, on a medium sized team with no meatshield. Hardly required, but nice to have.
3) See comments above. 
If you were to only allocate two more slots and not change anything else though, I'd recommend 1 in shadow fall (endrdx), and 1 in freeze ray (it's lonely).
The toon is still a lowbie, level 12 and it's, for the most part, a soloer.
I did put a lot of emphasis on recharge because it's fairly easy to build on Dark toons w/o sacrificing much at all (while boosting the tohit debuffs), it helps with stacking my debuffs and my hold and it helps keep my long recharge powers available; even w/o having to Hasten. (plus I'm kind of a speed junkie).
Clouded senses does this at less cost, fewer slots and is level maxed earlier than Positron's Blast .
*
I also went for some defense to supplement the tohit debuffs; hence the 6 slotting of Clouded Senses in Twilight Grasp. (Funny thing is that I was going to ask about utilizing the last to slots for more healing power as opposed to having some added defense. I went with the defense because the passive recharge is set to where I can offset any big green numbers by HPS (if that makes any sense). Plus I have Fluffy.
But being that I would have some pretty decent controls; I could switch those two slots into something a bit more green.
*
Fluffy, I'm holding on calling that my permanent slotting; I thought he could slot Clouded Senses as well (as part of a fix); I aslo figure that he uses his -tohit more than he uses his heal so that's what I would focus on for him.
I could drop one of the Recharge IOs from HT and move it over to Fluffy and give myself the same dilemma I had with TG; larger green numbers or more defense (if he can hold CS slotting).
*
Having Soul Drain is dependent on if my rain powers get a damage boost for it (and Aim); if they do not, then I'll scrap one or both for a second Hold power.
I do like having Power Boost for my tohit debuffs; so I doubt that I'd venture out of the Dark Patron Set; at least not for this build. Hehe, I forgot that I could prep my combo (and normal combat) with it... could really nix the need for 6-slotted CS sets.
*
I'm used to using HT as a debuffing tool and for assaulting huge mobs (like sharkhead miners) moreso than as a rez tool (I spend about 95% of my gameplay soloing).
Thanks for your input and btw, I always CTRL-C whatever I type in these forums... so if they do start acting wonky; then I can just paste it back.
Apparently, I play "City of Shakespeare"
*Arc #95278-Gathering the Four Winds -3 step arc; challenging - 5 Ratings/3 Stars (still working out the kinks)
*Arc #177826-Lights, Camera, Scream! - 3 step arc, camp horror; try out in 1st person POV - 35 Ratings/4 Stars
This is one of the builds I'm looking to get into. The emphasis, for this build, is on combat speed, debuffing and mob control.
I looking at pulling, as my 'big' combo:
Fearsome + Darkest + Tar Patch + Howling + Soul Drain + Aim + Ice Storm + Blizzard
The things I'm needing to work out:
1. Is Hasten overkill given the amount of global recharge listed?
2. In your opinion, between the tohit debuffs, fears and stuns... do I really need to push for more regular defense than I already have?
3. I'm sitting on 2 unallocated slots; suggestions?
Villain Plan by Mids' Villain Designer 1.401
http://www.cohplanner.com/
[u]Click this DataLink to open the build![u]
Ice/Dark 50: Level 50 Magic Corruptor
Primary Power Set: Ice Blast
Secondary Power Set: Dark Miasma
Power Pool: Speed
Power Pool: Fitness
Power Pool: Leaping
Ancillary Pool: Soul Mastery
Villain Profile:
Level 1: Ice Bolt <ul type="square">[*] (A) Devastation - Accuracy/Damage[*] (3) Devastation - Accuracy/Damage/Recharge[*] (7) Devastation - Accuracy/Damage/Endurance/Recharge[*] (17) Devastation - Damage/Endurance[/list]Level 1: Twilight Grasp <ul type="square">[*] (A) Cloud Senses - Accuracy/ToHitDebuff[*] (5) Cloud Senses - Accuracy/Endurance/Recharge[*] (9) Cloud Senses - ToHit Debuff/Endurance/Recharge[*] (11) Cloud Senses - Accuracy/Recharge[*] (40) Cloud Senses - ToHit Debuff[*] (42) Cloud Senses - Chance for Negative Energy Damage[/list]Level 2: Ice Blast <ul type="square">[*] (A) Devastation - Accuracy/Damage[*] (3) Devastation - Accuracy/Damage/Recharge[*] (7) Devastation - Accuracy/Damage/Endurance/Recharge[*] (19) Devastation - Damage/Endurance[/list]Level 4: Darkest Night <ul type="square">[*] (A) Dark Watcher's Despair - To Hit Debuff/Endurance[*] (5) Dark Watcher's Despair - To Hit Debuff/Recharge/Endurance[*] (15) Dark Watcher's Despair - To Hit Debuff[*] (17) Dark Watcher's Despair - Recharge/Endurance[/list]Level 6: Tar Patch <ul type="square">[*] (A) Recharge Reduction IO[*] (19) Recharge Reduction IO[*] (34) Slow IO[/list]Level 8: Aim <ul type="square">[*] (A) Adjusted Targeting - Recharge[*] (9) Adjusted Targeting - To Hit Buff/Recharge[/list]Level 10: Hasten <ul type="square">[*] (A) Recharge Reduction IO[*] (11) Recharge Reduction IO[/list]Level 12: Ice Storm <ul type="square">[*] (A) Recharge Reduction IO[*] (13) Recharge Reduction IO[*] (13) Damage Increase IO[*] (15) Damage Increase IO[*] (21) Damage Increase IO[/list]Level 14: Super Speed <ul type="square">[*] (A) Blessing of the Zephyr - Run Speed, Jump, Flight Speed, Range/Endurance[*] (40) Blessing of the Zephyr - Knockback Reduction (4 points)[/list]Level 16: Hurdle <ul type="square">[*] (A) Jumping IO[/list]Level 18: Health <ul type="square">[*] (A) Regenerative Tissue - +Regeneration[*] (43) Numina's Convalescence - +Regeneration/+Recovery[*] (43) Numina's Convalescence - Heal[/list]Level 20: Stamina <ul type="square">[*] (A) Endurance Modification IO[*] (21) Endurance Modification IO[/list]Level 22: Fearsome Stare <ul type="square">[*] (A) Cloud Senses - Accuracy/ToHitDebuff[*] (23) Cloud Senses - Accuracy/Endurance/Recharge[*] (23) Cloud Senses - Accuracy/Recharge[*] (27) Cloud Senses - ToHit Debuff/Endurance/Recharge[*] (37) Cloud Senses - ToHit Debuff[*] (40) Cloud Senses - Chance for Negative Energy Damage[/list]Level 24: Bitter Ice Blast <ul type="square">[*] (A) Cloud Senses - Accuracy/ToHitDebuff[*] (25) Cloud Senses - Accuracy/Endurance/Recharge[*] (25) Cloud Senses - ToHit Debuff/Endurance/Recharge[*] (27) Cloud Senses - Accuracy/Recharge[*] (31) Damage Increase IO[*] (31) Damage Increase IO[/list]Level 26: Shadow Fall <ul type="square">[*] (A) Steadfast Protection - Resistance/+Def 3%[*] (29) Steadfast Protection - Knockback Protection[*] (29) Luck of the Gambler - Recharge Speed[/list]Level 28: Howling Twilight <ul type="square">[*] (A) Endurance Reduction IO[*] (31) Endurance Reduction IO[*] (37) Recharge Reduction IO[*] (37) Recharge Reduction IO[/list]Level 30: Combat Jumping <ul type="square">[*] (A) Blessing of the Zephyr - Run Speed, Jump, Flight Speed, Range[*] (36) Blessing of the Zephyr - Run Speed, Jump, Flight Speed, Range/Endurance[*] (36) Luck of the Gambler - Recharge Speed[*] (42) Kismet - Accuracy +6%[/list]Level 32: Blizzard <ul type="square">[*] (A) Cloud Senses - ToHit Debuff[*] (33) Cloud Senses - ToHit Debuff/Endurance/Recharge[*] (33) Cloud Senses - Accuracy/ToHitDebuff[*] (33) Cloud Senses - Accuracy/Endurance/Recharge[*] (34) Damage Increase IO[*] (34) Damage Increase IO[/list]Level 35: Freeze Ray <ul type="square">[*] (A) Lockdown - Accuracy/Hold[*] (36) Lockdown - Accuracy/Recharge[/list]Level 38: Dark Servant <ul type="square">[*] (A) Dark Watcher's Despair - To Hit Debuff/Recharge/Endurance[*] (39) Dark Watcher's Despair - To Hit Debuff/Endurance[*] (39) Dark Watcher's Despair - To Hit Debuff/Recharge[*] (39) Dark Watcher's Despair - To Hit Debuff[/list]Level 41: Dark Embrace <ul type="square">[*] (A) Resist Damage IO[*] (42) Resist Damage IO[*] (43) Impervious Skin - Status Resistance[*] (46) Impervious Skin - Resistance/Endurance[/list]Level 44: Soul Drain <ul type="square">[*] (A) Adjusted Targeting - Recharge[*] (45) Adjusted Targeting - To Hit Buff/Recharge[*] (45) Obliteration - Accuracy/Damage/Endurance/Recharge[*] (45) Obliteration - Accuracy/Damage/Recharge[*] (46) Obliteration - Damage[*] (46) Gaussian's Synchronized Fire-Control - Chance for Build Up[/list]Level 47: Power Boost <ul type="square">[*] (A) Recharge Reduction IO[*] (48) Recharge Reduction IO[/list]Level 49: Summon Mistress <ul type="square">[*] (A) Expedient Reinforcement - Accuracy/Recharge[*] (50) Expedient Reinforcement - Accuracy/Damage/Recharge[*] (50) Expedient Reinforcement - Endurance/Damage/Recharge[*] (50) Expedient Reinforcement - Damage/Endurance[/list]------------
Level 1: Brawl <ul type="square">[*] (A) Empty[/list]Level 1: Sprint <ul type="square">[*] (A) Empty[/list]Level 2: Rest <ul type="square">[*] (A) Empty[/list]Level 1: Scourge
------------
[u]Set Bonus Totals:[u]<ul type="square">[*]19% DamageBuff(Smashing)[*]19% DamageBuff(Lethal)[*]19% DamageBuff(Fire)[*]19% DamageBuff(Cold)[*]19% DamageBuff(Energy)[*]19% DamageBuff(Negative)[*]19% DamageBuff(Toxic)[*]19% DamageBuff(Psionic)[*]3% Defense(Smashing)[*]3% Defense(Lethal)[*]3% Defense(Fire)[*]3% Defense(Cold)[*]9.88% Defense(Energy)[*]9.88% Defense(Negative)[*]3% Defense(Psionic)[*]3% Defense(Melee)[*]16.8% Defense(Ranged)[*]3% Defense(AoE)[*]9% Max End[*]56.3% Enhancement(RechargeTime)[*]10% FlySpeed[*]80.3 HP (7.5%) HitPoints[*]10% JumpHeight[*]10% JumpSpeed[*]Knockback (Mag -8)[*]Knockup (Mag -8)[*]MezResist(Confused) 20%[*]MezResist(Held) 20%[*]MezResist(Immobilize) 20%[*]MezResist(Sleep) 22.2%[*]MezResist(Stun) 22.2%[*]MezResist(Terrorized) 20%[*]6.5% (0.11 End/sec) Recovery[*]36% (1.61 HP/sec) Regeneration[*]10% RunSpeed[/list]------------
[u]Set Bonuses:[u]
[u]Devastation[u]
(Ice Bolt)<ul type="square">[*] 12% (0.54 HP/sec) Regeneration[*] 24.1 HP (2.25%) HitPoints[*] 3% DamageBuff(All)[/list][u]Cloud Senses[u]
(Twilight Grasp)<ul type="square">[*] Status Resistance 2.5%[*] 2.25% Max End[*] 6.25% Enhancement(RechargeTime)[*] 5% JumpSpeed, 5% JumpHeight, 5% FlySpeed, 5% RunSpeed[*] 3.75% Defense(Ranged), 1.88% Defense(Energy), 1.88% Defense(Negative)[/list][u]Devastation[u]
(Ice Blast)<ul type="square">[*] 12% (0.54 HP/sec) Regeneration[*] 24.1 HP (2.25%) HitPoints[*] 3% DamageBuff(All)[/list][u]Dark Watcher's Despair[u]
(Darkest Night)<ul type="square">[*] 16.1 HP (1.5%) HitPoints[*] 2.5% (0.04 End/sec) Recovery[*] 5% Enhancement(RechargeTime)[/list][u]Adjusted Targeting[u]
(Aim)<ul type="square">[*] 2% DamageBuff(All)[/list][u]Blessing of the Zephyr[u]
(Super Speed)<ul type="square">[*] 3.13% Defense(Ranged), 1.56% Defense(Energy), 1.56% Defense(Negative)[*] Knockback Protection (Mag -4)[/list][u]Numina's Convalescence[u]
(Health)<ul type="square">[*] 12% (0.54 HP/sec) Regeneration[/list][u]Cloud Senses[u]
(Fearsome Stare)<ul type="square">[*] Status Resistance 2.5%[*] 2.25% Max End[*] 6.25% Enhancement(RechargeTime)[*] 5% JumpSpeed, 5% JumpHeight, 5% FlySpeed, 5% RunSpeed[*] 3.75% Defense(Ranged), 1.88% Defense(Energy), 1.88% Defense(Negative)[/list][u]Cloud Senses[u]
(Bitter Ice Blast)<ul type="square">[*] Status Resistance 2.5%[*] 2.25% Max End[*] 6.25% Enhancement(RechargeTime)[/list][u]Steadfast Protection[u]
(Shadow Fall)<ul type="square">[*] 1.5% (0.03 End/sec) Recovery[*] 3% Defense(All)[*] Knockback Protection (Mag -4)[/list][u]Luck of the Gambler[u]
(Shadow Fall)<ul type="square">[*] 7.5% Enhancement(RechargeTime)[/list][u]Blessing of the Zephyr[u]
(Combat Jumping)<ul type="square">[*] 3.13% Defense(Ranged), 1.56% Defense(Energy), 1.56% Defense(Negative)[/list][u]Luck of the Gambler[u]
(Combat Jumping)<ul type="square">[*] 7.5% Enhancement(RechargeTime)[/list][u]Cloud Senses[u]
(Blizzard)<ul type="square">[*] Status Resistance 2.5%[*] 2.25% Max End[*] 6.25% Enhancement(RechargeTime)[/list][u]Lockdown[u]
(Freeze Ray)<ul type="square">[*] 3% DamageBuff(All)[/list][u]Dark Watcher's Despair[u]
(Dark Servant)<ul type="square">[*] 16.1 HP (1.5%) HitPoints[*] 2.5% (0.04 End/sec) Recovery[*] 5% Enhancement(RechargeTime)[/list][u]Impervious Skin[u]
(Dark Embrace)<ul type="square">[*] MezResist(Sleep) 2.2%[*] Status Resistance 7.5%[/list][u]Adjusted Targeting[u]
(Soul Drain)<ul type="square">[*] 2% DamageBuff(All)[/list][u]Obliteration[u]
(Soul Drain)<ul type="square">[*] MezResist(Stun) 2.2%[*] 3% DamageBuff(All)[/list][u]Expedient Reinforcement[u]
(Summon Mistress)<ul type="square">[*] Status Resistance 2.5%[*] 3% DamageBuff(All)[*] 6.25% Enhancement(RechargeTime)[/list]
Apparently, I play "City of Shakespeare"
*Arc #95278-Gathering the Four Winds -3 step arc; challenging - 5 Ratings/3 Stars (still working out the kinks)
*Arc #177826-Lights, Camera, Scream! - 3 step arc, camp horror; try out in 1st person POV - 35 Ratings/4 Stars