[DiapDealer]

As far as I'm concerned, Sigil is handling filenames with ampersands correctly. You can rename files within the epub to contain ampersands and Sigil will properly encode the path in the opf manifest. Sigil will also open any epub that has a filename with an ampersand so long as the ampersand is encoded correctly in the opf manifest (and the path exists) to begin with.

The only time there's a problem is if an existing epub has an opf with an invalid ampersand character in it. And even then, it's not because of the illegal ampersand character (directly) that files might be discarded when subsequently saving the epub. It's the fact that the illegal character makes the path to the file incorrect. Which means it's not properly manifested. And improperly manifested files have always been discarded when opening them. It's no different than a mispelling or a case sensitivity issue.