Good questions!
--> Can a follower be placed on a tower foundation without any tower piece, or can a follower only be places on one or more tower pieces? The rules are not so precise about this: "to finish the construction of a tower by deplying afollower on top of it".
I think here there has to be a tower block in play. Apart from anything else, how would you indicate that the follower was stood on the tower foundation, rather than the city next to it?
--> If the count & river is combined, you start "by placing the spring in such a way that it leads away from the city". How is this done? as there are more possibilities to do this, Or who decides how this piece is placed? as the game is only supposed to start by putting the next river tile or the fork.
The starting player decides where and how. That's the official rulling; instead, we normally place the river first, and then try to fit Carcassonne City to that. But see my next comment.
--> What happens when a river get "stuck"? because the rulings not to make immediate 180 degree turns, and the 2 river branches may not be connected, is not sufficient to guarantee a free river.
True, the rule doesn't guarantee a free river - it just reduces the likelihood of an impass. If you reach an situation where you can't complete the river, take all the tiles off and start again. In fact, you should only really have such a problem through poor or malicious placement of the tiles - and general consensus is that a player who continues to wilfully obstruct play in that way should be dealt with severely (defenestrated, in my view;D)