Gimp, MyPaint, etc. can be used with the tablet ONLY.
Guidelines in Gimp still don't work as long as the pen is configured as it should be.
BUT, I think I just stumbled over something interesting:
Say in Gimp, when stylus and eraser are set to "screen" the behavior is as described above. However, when only the stylus is set to "screen", while the eraser is "disabled", then everything works as expected: draw, select, crop with the mouse, and guidelines are working.
What's funny, now MyPaint works with the mouse as well! O.o