In other words, an ".epub" file is just a ".zip" file with a few additional metadata files added. Software that purports to "convert" HTML to .epub should not do /anything/ to the source file, except perhaps to insure that it is valid XHTML (for older HTML files). There is no need to validate an .epub conversion, as no conversion should have occurred. If a rendered .epub document does not look exactly like the same collection of files rendered by a browser from the file system, it is the fault of the .epub rendering software, not the "conversion."
You make an interesting thesis, which, rare in the case of DP/PG arguments, is eminently testable. I have done so, and you clearly have not. Take a PG HTML zip file, say "76" for the sake of completeness. Download it, and unpack it on your computer. Take a PG epub "zip" file, say pg76.epub for concreteness. Download it, and unpack it on your computer. Now, look at the contents. Do they have the same HTML files? No they do not. Do the have the same number of HTML files? No they do not. Are the contents of the HTML files identical? No they are not. For the sake of completeness, open the first HTML file of each. Do the files RENDER the same on your browser when you actually TRY them to see if your thesis is correct? No they do not RENDER the same. It is an interesting thesis that PG epub files are "just" a zipped version of the PG HTML files -- but it is an easily demonstrably false thesis. Marcello's epub software does more than "just" pack the HTML files into an epub package. Ask him for a copy of his converter software, and see what the conversion actually entails. And/or ask Marcello what conversions he actually does to move from the HTML version to the epub version. Thus again, I suggest that it would be a good idea to have a portable version of Marcello's epub conversion software that we could use for testing on our local machines. Given a portable version of the epub conversion software going to mobi is easy using the same Amazon/Mobipocket provided epub->mobi conversion software that Marcello is already using.