The fact that ePUB is not the same as mobi makes the fact that it validates non-valid.
I believe that floats are not possible within mobi, but I am not quite sure. Calibre should be able to create a TOC via the toc.ncx. Mobi doesn't use the toc.ncx, but needs a separate file for that. In the new Sigil version you can create a toc.html based upon the toc.ncx.
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