[resolfe]

I tested the 2 mentioned tools but they do not fulfill my requirements, e.g. they partly change the formats/styles of the text.

So I will stick with Calibre (which basically does everything what I want/need), I think I just need some last tweaks in order to get the result I want.
In regard of the image size in the output files, I will prepare 2 different word files with different image sizes (one for the epub and one for the mobi, since they interpret the image size differently).