Many thanks for your helpful hint, Ceciliabr,

The boxes to circumvent issue 1 and 2 I already used before my post, but I must admit that I don't like this very much. If it would be only 1 or 2 times, ok, but more than 30 times is too much. I prefer to have objects (furniture and other stuff) separated from construction items as walls, textures, windows etc.

I would really appreciate if theses issues could be fixed in one of the next releases.

With regard to issue no. 3 I must admit that I am still confused.

Ceciliabr, how should a box which has 90° angles help to build the steps of my stairs which do not have a rectangular shape?
Furthermore the box from SW3D has texture issues as well, when you rescale it. The texture gets compressed on those areas which are getting smaller. I would appreciate some more help on this.

Issue no. 4: Great idea, which I didn't find myself. The only little disadvantage is the visible cutting line which separates the two parts of the wall.

So I still looking for some help on issue no. 3. Thank you.