Seems to me like you've pretty much got it. The low level squishitude is standard for Stalkers. It certainly gets better.
I don't remember the specifics, but you have a chance for your attacks to crit outside of hide, and that chance goes up for every team member within a certain radius of you. You have a base chance to crit even when solo though.
Your role on teams is up to you, and it can change depending on circumstances. If you have a Brute, Dom, or Mastermind who is taking care of the alpha, you may want to open with an AoE to help take down the spawn faster. If no one else is up to the task, or if you just like rushing ahead, you can handle that job as well. What allows you to do that, aside from your defensive secondary, is the demoralize effect attached to AS. In order to get the effect the target needs to survive the AS, so you just AS a boss, or a lieutenant without Buildup.
For primary I'd recommend Elec. It's great fun and the AoE in it is perfect for teaming. When you just want to do mass AoE damage opening with Lightning Rod->Thunderstrike is nice damage and mitigation through knockdown. Other times you have AS for hard targets. It's my favorite set for a team-focused Stalker.
Spines is nice too, but the long animations and lack of good single-target in the lower levels, along with the more resisted damage type makes it my second choice for AoE. That's not to say you shouldn't choose it if you prefer it. Both sets are good. My answer was to roll one of each. 
Oh yeah Dual Blades has some good AoE too. That one I haven't tried personally because it just hasn't captured my interest at all, but it's another option if you like weapons sets and don't mind melee cones.
Welcome back!
I like Ninja Blade/Ninjitsu. It is very Solid, but little AOE. Damage is very good though.
As far as Role, Stalkers can provide many services for a team.
You can Stealth Missions, and TP the Team to the Boss/Blinkie for quick finishes.
You can scout, and tell the team what kind of Mob is waiting for them around the corner.
AS is awesome for taking out Troublesome Enemies; Tsoo Sorcerers, Air Raider Operations Engineers, Malta Sappers, etc...
The Run and Hide playstyle is less used now, and I as a Stalker prefer to stay and fight.
It all depends on your team though, and how they are playing missions. If they are "rush in and kill them all fast" teaming, then just depend on your damage.
I love my Stalker, and have never heard one complaint that I was on a team. I have recieved Tells asking me to join teams to help them complete difficult missions, lol. I can dish out more damage than some Brutes, and take the Alpha Strike with no problems.
Stalkers are very fun to play.
Just started another one, lol.
Black-Strike: lvl 50 AR/Dark Corruptor
Brutally Beautiful: lvl 50 BA/Inv Brute
Seared Earth: lvl 50 EC/EA Dominator
Yashi Onuku: lvl 50 Ninja Blade/Ninjitsu Stalker
Death-Widow: lvl 50 Night Widow
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I don't remember the specifics, but you have a chance for your attacks to crit outside of hide, and that chance goes up for every team member within a certain radius of you. You have a base chance to crit even when solo though.
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Stalker
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If the Stalker isn't Hidden and he's attacking a PvE enemy, his chance of critting is 10% + 3% per teammate within 30'.
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"All right, they're on our left, they're on our right, they're in front of us, they're behind us...they can't get away this time"- Chesty Puller US Marine Corps
One of the god thing about this game is that the roles aren't always set in place like some other MMOs. It's kind of flexible depending on your powers, you know?
That being said, most Stalkers on teams consider their role to be "Take out the biggest annoyance in the mob with AS, protect the squishies, and clean up the leftovers."
As for your question on /Nin Stalkers, I think that the current most popular one is Dark Melee/Ninjutsu. Dark Melee's attacks have a -ToHit Debuff that makes your defense more effective against the enemies you hit, and its powers also have a melee attack with a self-heal (Siphon Life), a melee immobilize with solid damage (Midnight Grasp), and a Melee fear (Touch of Fear) what can be used to thin the number of enemies attacking you. In fact, you can stack Touch of Fear and even Fear an Elite Boss (I LOVE doing that, too!). It also deals Negative Energy damage, which many enemies don't have protection against.
The biggest drawback to a Dark/Nin Stalker is the lack of AoE damage. Most of its attacks are Single Target only.
Ninja Blade/Ninjutsu is also popular because of Divine Avalanche, a weak attack that adds 15% to your melee defense, and can be stacked to give you a whopping 30% melee protection on top of your armors. It has a bit more AoE than Dark Melee, as well.
From what I've seen, you can't go wrong with either. I personally play a Dark/Nin stalker, and I've managed to solo Mako, Positron and Vakyrie as AVs (though they sure as all heck weren't easy, and Mako killed me 3 times before I beat him). I have a buddy who has a Ninja Blade/Ninjutsu Stalker who soloes EBs pretty easily, too.
Too many alts to list.
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I don't remember the specifics, but you have a chance for your attacks to crit outside of hide, and that chance goes up for every team member within a certain radius of you. You have a base chance to crit even when solo though.
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Stalker
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If the Stalker isn't Hidden and he's attacking a PvE enemy, his chance of critting is 10% + 3% per teammate within 30'.
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I just noticed that article has an error in it.
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Stalkers cause a great deal of additional damage through critical hits. The exact chances of a critical vary greatly with the situation and the power used. Stalker criticals do double damage except in one case which we'll get to in a moment.
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It goes on to talk about AS that breaks the double damage rule, but Energy transfer and Total focus both break the rule as well. Total focus only gets a crit on it's smashing damage, and ET doesn't cause self damage if it crits. Maybe someone with an account there can fix this.
Dirges
Hey all, coming back to the game after two years (Lost my old account). I'm interested in making a melee villain, but the "gotta move" mentality of a Brute is too much for me. So, I'm looking for a nice stalker. If I understand correctly, Stalker's attacks now have a change to crit, OUTSIDE of hide, which seems like a pretty nice boost.
Also, like to team a lot, and have been wondering what role Stalkers usually take on, and any tips on how to approach battles (solo and teamed). It seems I'm always face planting in the early levels... Lastly, any suggestions on a good primary to go with /ninja. I've been reading around the forums, and it seems like a good AoE primary would work best when teaming? I know elec and spines are good AoE primaries, but not very familiar with the rest. Any and all tips are appreciated. Thanks!