What happens to troops that cant be placed in cohesion?

Let's say the other player repositions your unit leader into a tight spot

Would the need to keep cohesion limit the other player from schenangins?

We already had this discussion once. If I remember correctly, there are no rules concerning this problem.

I would rule that the move, that made cohesion impossible, is not allowed. Although the move might be legal, it would lead to an illegal game state. Therefore the whole move is illegal.

There are a few corner cases that I know of that the rules don't cover. One is what you outlined above and the other is what to do if you can't fit a unit in your deployment zone for whatever reason.