Until now, the ceiling computing works for simple slopping walls only.

..and that's a great feature and far more than I would have expected. Now that I found out that I simply have to disable that for the rooms under the roof, the export obj > kmz > sketchup works great (direct way obj > sketchup with the obj-plugin works also but wrong orientation, you have to rotate the whole model).

I have to repeat it: once the model is in Sketchup you can do lots of useful things that are important:
- great 3D navigation (walk, look around, Hand-tool) to view the house and rooms from all perspectives
- print every view with good quality (chose "extreme quality" in page preview dialog)
- simulate sun and shadows at all times (morning/evening, summer/winter)
- still trying with Blender to render HQ picutres

- cons: textures are not imported, only colors

For both Sketchup and Blender export, I would find the following useful:

I suggest the following feature: when selecting the "export to obj-Format", you have the option "export furniture as boxes". If this option is set, all movable items (not windows, stairs, doors) will be replaced by a simple box with the same dimensions.

Any chances for that?