Sasha is stated to be entirely Human, using only a nano-machine suit, while the three Rin sisters all have cybernetic feet (for what reason, i will never know).
Maybe Ai Rin just wanted to kill someone because she's fun like that, Fei Rin didn't bother to stop her since she hates the B.P.S., and they both just grasped an easy excuse to explain it away to Rin Rin later?
Did Nikolai die at the end? He's cut in half, but the other characters talk about him as if he's still alive and even go to put him on trial.
Death seems like more of an inconvenience to people as far up the ranks as him. Considering his head was the only organic part of him left, it's likely they'd just revive him for his trial and punishment.
Likely his head was organic and his body was alive, just now a stump. Death is a real thing in Anarchy Reigns, or else Stela and Ondine's deaths wouldn't have been as driving as they were.
The game really could have used a little Chekhov's Gun to set up the idea (Perhaps collecting Durga's head to stand trial?), but in settings with Transhuman elements it's not too unusual for characters to survive being reduced to a brain and being reconstructed at a later date, even if they've flatlined. Hell, look at RoboCop.
There is an All There in the Manual explanation. In Big Bull's bio, it states that he elected to have his brain taken out of his body and into the minotaur mecha we know and love, and Garuda was a dead man who had his brain copied onto a memory card and slotted into a customised Gargoyle. Even if Nikolai was dead, they could just copy his brain onto an SD card and put him in a coffee machine while they put him on trial.
What's the significance of Big Bull's symbols on his head and chest? Sure, one means "blind spot" and the other is the emblem of Neptune (planet and Greek God). Is it just something Bull did for style or what? This is a minor point, but no amount of research explains it.
Probably is for style. The blind spot became one of his gang's symbols, since it's on the back of their jackets, so who's to say.