@Hans
1.I agree it's not convenient and not easy to guess the angle of rotation. Let's forget about this solution.
2. Seat is not supposed to rotate - the idea was to force horizontal position for folded "legs" too. But folded chair also needs rotation, only this time it's x90degrees.
3. Yes, okh's suggestion turns out the best.

@UbuntuBirdy
3 works as I intended - can you show screenshot of these nauhty hinges?

@Emmanuel
Apparently I can't get out of my head rails and hinges [;)]
Here is something you can spin and spin...