@Peter. I think the compare idea is valid. I know of no way of comparing epubs except unzipping them and comparing the htmls. I've got Beyond Compare and like it a lot.But for a quick job I like QuickDiff.
http://www.quickdiff.com/
@Dennis I don't use Sigil to edit per se. But it just works out that way. I can edit the document in .odt till I'm blue in the face. So I convert it to html and save it to Sigil. Well, I'm gonna find stuff to tweak there. Until I drop the thing on Amazon I'm going to find stuff to tweak.