I don't believe there is any sort of "approved html". But some people may be able to give you tips on what worked for them.
Yes, footnotes should link both ways. They don't have to, but it would be nice.
Suggestion -- don't use superscripted numbers, use "[1]" etc. instead. It is easier for us to click.
If you can get ahold of an original DOCX (not old DOC) then Toxaris' Word Addin can do a much better job of exporting an actual base EPUB than Word itslef will do with filtered HTML. Alternatively, the
calibre E-Book Editor can directly import DOCX, again with much better results than exporting from Word.