I just attempted this. I believe it IS possible, outside of killing Nikoladze of course (btw, you can knock out Grinko and pass the level, so yeah, only Nikoladze must die). I just completed a run where I only killed maybe 6 people (3 in Abattoir).
I could have attempted to not kill anyone at the end of Abattoir but I really didn't even wanna bother trying as I only had a couple ring foils left. Trying to save the soldiers AND Chinese ambassador at the same time while getting shot at would have been pretty goddamn annoying. And there's too much light and too many walls to get a clear shot with a non-lethal or sneak up on anyone. But I'm sure if I really tried a bunch of times I could have done it.
I also killed two people in Kalinatek by accident. After I saved the people beside the wall mines, two guards just showed up in the doorway as I went to leave to disarm the bomb. They caught me by complete surprise and I instinctively and, normally I would say fortunately, but this time I didn't want to... shot him in the head with my pistol pretty much right as I withdrew it. It was a hell of a shot with a bit of luck involved, heh. Then I was like.. ahh screw it I'll kill the other one too.
Presidential Palace was the hardest level to do this outside of Abattoir.
Let me preface this. I don't know if anyone reading this is aware, but the main trick to doing a no-kill run is to use sticky cameras. Shoot them in the head with a sticky camera and it knocks them out. You really have to practice with this as you have to lob the heavier shots a bit, but I've played this game a bunch of times and used the trick a lot in CIA and Chinese Embassy 1 so I've gotten pretty skilled at lobbing them at people's heads from 20 feet away. Why is this so useful? Because you can pick sticky cameras back up. Infinite nonlethal ammo.
So anyway... this trick is completely ineffective against the special forces guards as they wear helmets. So I had to completely avoid these guys or sneak up on them and grab them, which is not easy because it seems like they can almost see in the dark... Normal guards, they'll pass by within 5 feet of you. I can't get within 10 feet of the special forces. I ended up using most of my non-lethal ammo when they come when you get out of the elevator as they know exactly where you are, even if you shoot out all the lights and move around silently, they still track you. So then I had like NONE left for the basement part. I had to redo the basement part probably 20 times before I figured out a good strategy of knocking out the first guard and then immediately running along the left wall to avoid them. You still have to knock another one out in the doorway while the ones you ran away from are shooting at you... mega-difficult, probably the hardest part of the game to do without killing besides maybe the end of Abattoir which I didn't even attempt.
SUMMARY: It's possible. Hardest parts are the end of Abattoir and basement of Presidential Palace. If anyone can post a good strategy for Abattoir I'd love to hear it.
"This is all we have... when we die.."