Mass-effect – Do I get the credit for the damage and kills the sabotaged enemies do

What you're dealing with is basically just the fact that it's a slower payoff than a melee specialization.

Sticking with the cloaking and sniping tactics will payoff. You'll have to invest a lot into building your rifle up, but eventually most enemies will be one-shot kills unless they have multiple layers of shields.

Of course there is a lot more in the way of hybrid options in ME3 than there was in ME2 so some of it will be up to you and how you play the character.

Cloak + Shotgun = AMAZING!

Right now my load out is: Mantis V Wraith V Shuriken V

Cloak and Operational Mastery are at 6 and Fitness and Disruptor Ammo are at 5. Everything else is at 1 until I max the others out.

I played around in a solo-multiplayer session a bit and found the reason why Sabotage seems to fail sometimes. The duration of Sabotage decreases significantly each time you apply it on the same enemy. So the first time you sabotage a specific enemy you get the full duration, but if the enemy survives that and you sabotage him again, the duration will be much shorter than the first time. It seems to be roughly halved each time you reapply sabotage.

After 4 times sabotage seems to lose any effect on that specific enemy, and this is not specific to Geth Primes or other superior enemies, you can observe that with any Geth trooper. The reason it seems to happen especially often with Geth Primes is that most other enemies just don't survive long enough so that you have to use sabotage that often on them.