If I play multiple Possessions in my turn (for example due to KC), and another Possession in the turn I control my opponent, when will he possess the third player/me, will my other Possession turns still happen?
Oh boy. Such questions are tricky, but this is what would happen.
You play Possession x2. At the end of your turn, the player to your left takes an extra turn in which you control him. During that extra turn, he plays Possession, because you made him for some reason.
Now, at the end of that turn, 2 different things are about to happen at the same time... he should take an extra turn from your previously played Possession, and the player to his left should take an extra turn from the newly played Possession. As it is his turn,
he gets to decide what order they happen in, but as you are possessing him, you make that decision for him. So you get to choose which happens first. his extra turn will happen first.
But no matter which one you choose, the same 2 things will end up happening, He'll get 1 more extra turn possessed by you, and then the player to his left will get an extra turn possessed by him. After both those extra turns are done, he will continue with his normal turn.