I too do Word docs saved as filtered HTML. I compile through Mobipocket Creator to check the file through the Kindle for PC app then upload the .prc to KDP and check it again in the previewer.
If I need a .mobi file (or an .epub), I use Calibre to create it from the .prc.
Using this method, I published out 20 books through KDP just in the past week (no, they weren't all mine

). I use title page graphics, links and graphics in the back matter, and mixed styles throughout (indents, no-indents, centering, heads, variable spacing between paragraphs, etc). One of the books even has a tricky custom table of contents. My interiors generally come out clean and relatively attractive.