1) I assume that a mech can not see me in a forest (because I may hide there).
2) I assume also that a mech can't see "behind" a forrest in the clear hexes.
Incorrect. Thematically, the mech towers over the forest, and cannot see you inside, because you hide under the crowns of the trees. Once you venture outside again, they can see you there.
First ever preview: Once I get around to making the sequel/expansion, it will contain new types of terrain; one of them will be mountains (or something like this), which you cannot see anything behind.
3) What happens if I´m in the forrest (assuming the frist to assumptions were right) sirectly in front of the mech and I try to enter the hex of the mech through his front hexside ?
I assume his auto defense starts immediately without any battle, right ?
That is absolutely correct.
Hope you have fun with the game. If you run across any other questions, please feel free to ask - I'll do my best to give a coherent answer