I'll flash the custom firmware tomorrow nevertheless, even though the buttons are not in the best condition. This part will be easy! I know computers!
Looks like my dream of refurbishing my favorite game console without any new parts failed. At least I didn't have to buy the funky screwdriver, a friend let me borrow it.
2/2
Hacked my 3DS. Adding some files on the SD card and pressing some buttons is much easier, than prying the funky connectors and driving some screws.
Installed a #uxn emulator. After changing ' to " in the source code, my Tic-Tac-Toe ROM compiled (who keeps changing the language?). It works, see the attached photo.
Also looks like I've damaged the bottom screen haha. Haha...
3/2