Update: The right enter key works as expected. I confused this with a bug in the relevant X Client.

Also, Ctrl + Alt is not recognized as AltGr, so for example, Ctrl + Alt + 7 should print an opening brace ({) but, depending on the application, either prints a seven (7) or nothing instead.

Sorry for brevity. If there is any chance to fix this issue, I will be very happy to assist you with further information! Until now (as a complete outsider to this project), I have only found out that I can modify xorg-server/xkeyboard-config/symbols/de to change the keyboard mapping (as a workaround, I have changed dead_circumflex for key <TLDE> to asciicircum), but I don't see where the AltGr key is recorded. Any help will be appreciated!

Best,
Christoph