Thanks a lot. Yes, adding the library.properties did indeed solve both of these problems.
However, I am facing a strange unrelated issue now: I have some menu entries in the board.txt. These don't properly show up. So I tried to approach this issue step by step by commenting stuff in boards.txt. It turned out, that these menus are cached somewhere. Once a board.txt has been parsed further changes in the menu structure don't show any effect.
I already tried deleting ~/.arduinoIDE and /tmp/arduino* but the menus simply stay the same when I reload the IDE. Are these cached? Where?
Edit: Deleting ~/.config/arduino-ide does remove the cached entries