Thank you for your feedback.
Sounds like there are some problems with Ubuntu 11.10. I just tried under Ubuntu 11.04 and didn't noticed the problems you reported about shorcuts, drag & drop and furniture handles. Can you check everything works when you use menu items?

When I draw a simple house with two floors (= two layers), and then use Modify layer to increase the thickness of the ceiling and decrease the height of the walls of the top layer, there is a "strange" overlap visible in the walls, that corresponds with the decrease of wall height.

You should decrease the height of the lower level walls also. Wall heights are independent from level height. This has some drawbacks like the one you reported but lets you also completely free to manage walls the way you want, something required to manage intermediate levels. Level height is actually only used to compute ceilings.

The compass has moved to the "old" position of 0,0.

I guess you edited an old document. On a new document, the location is at the same place.

When level 1 is active, it's still possible to delete furniture on level 0.

I couldn't reproduce this bug, except if the selected piece is high enough to appear on a higher level, in which case I found normal to be able to select it and delete it.