1st.
There is no “Change Controls” from main menu, only from ingame pause menu.
2nd.
Rebind an already bound key opens a popup with 2 choices, both labeled with “Apply changes” - I assume, the left on is “OK” and the right is “cancel”. (English and German, other languages probably too)
3rd.
A bound key to “Rotate Building Opposite” isn’t shown in the controls list and doesn’t work ingame, but if you assign that key to another function the warning popup says “Is already bound to Rotate Building Opposite”.