1. I like the demon whip attacks. There is more than the usual crush for taking great powers in a mastermind set with dark miasma. In such instances, I would prioritize crack the whip. Stealthing in with shadowfall, crack the whip can get a lot of attention to your mastermind since it is a fairly wide cone. I didn't respec out of the whip powers on my demon summoning masterminds.
2. Having Dark Miasma for a secondary, I would definitely build towards recharge and recovery.
3. Living Hellfire is definitely a keeper. I cast it only on the 2nd and highest tier demons. In general I like it on my demon prince, but if there is a situation where my 2nd tier demon is having a rough time, I cast on him to help cushion his aggro. The little imps do a great distraction job for your buffed pet.
4. If your mastermind draws aggro at range, the minions will all deliver a volley of ranged attacks and then move into melee in general. The fire demonling almost always stays at range, he has more ranged attacks. Your cold demonling almost always moves to melee quickly; he cycles through his few range attacks quickly, being ice blasts, and then moves into scrapper mode. Being in melee with a demon with a heal aura and shield aura, with a second demon with a damage aura, along with the demon prince with a slow aura isn't a bad thing at all when you are packing an AoE twilight grasp, tar patch, darkest night, etc. Adding the dark servant to the mix, just makes sick for your opponents.
Triumphant Defenders Forever
Psylenz FF/Psi, ArticQuark Storm/Rad, Symon BarSisyphus Bots/psn, Max VanSydow Thugs/Dk, Cyclone Symon Bots/stm, Blue Loki Ice/Cd, Widow 46526
HelinCarnate:OMG it is so terrible. I have the option to take 3 more powers but no additional slots. Boo F'ing hoo.
That's a lot of help Psylenz, thanks very much. 
@Dante EU - Union Roleplayer and Altisis Victim
The Militia: Union RP Supergroup - www.themilitia.org.uk
I'll only chime in for #4 in the OP. Psylenz did a great job summarizing.
My DS/dark MM is my 3rd MM to 50, after a bots/storm and thugs/PD. And for the DS primary, I had to pay attention a lot more to binds and controlling pets. This was something that caught me off gaurd from my previous 2 MM's. The biggest offenders IME was the cold tier 1 and the hellfire tier 2. They scattered practically every spawn of mobs and I had to pull them back in constantly or they would just get killed. Psylenz answered why that cold tier 1 runs off so fast. I assume the tier 2 has some cycling issue too.
And second, I admit another new learning point was that dark has the aoe heal the other 2 with ST heals. I thought I was use to this since I have a D3 defender. I had to move around more to make sure in range (and fluffy helped tremendously later on).
Hi folks.
Even though I've had access to them for a number of months now, I've never felt the compulsion to try out Demon Summoning. I think perhaps being a melee toon on a team full of screaming cowbeasts kind of put me off a bit. However, with GR imminent, I realised that I had the perfect opportunity to reroll one of my main characters as a heroic DS/Dark Mastermind. I've browsed through the forum and found a few answers to my many questions but there's some I still haven't got a satisfactory answer to. I hope you can help.
1) I'm used to skipping MM attacks apart from theme mostly due to them being a bit naff. However, I gather that the whip attacks do actually have some virtue in debuffing the foe a little despite probably being low damage. Are they all worth getting? Or do some stand out from others? Is the knockdown reliable? I'm used to being in bodyguard mode and getting into melee so the short range doesn't bother me at all. Or are they the type of thing which are useful while levelling but get respecced out later on?
2) When planning my build, what should I be building for? Melee defence, range, recharge speed? I'm not a min/maxer but I do like to think ahead and I'm not quite sure what I'm aiming for with this one.
3) Living Hellfire. Worth it? The buff seems nice but a little short lived. Would I be a fool for skipping it?
4) The current 'bug/no it's not a bug' situation aside, do demons like melee or range? I figure the former. I'm used to using the Numberpad Keybind system so issuing orders is easy, I just want to know where they're most efficient.
Thanks as always!
@Dante EU - Union Roleplayer and Altisis Victim
The Militia: Union RP Supergroup - www.themilitia.org.uk