If you use DOCX properly, i.e. styles etc. then
@Toxaris' MSWord Addin does a very nice job of exporting as EPUB where the experts can fine-tune as needed.
calibre's editor can also natively open DOCX and convert it to EPUB for editing and saving.
I do recommend that the people who know HTML+CSS clean up all the books, perhaps a division of labor would be appropriate.