@Jon: I tried WinRAR late last night, using it the same way I'd use WinZip. The ePub file came out fine for me, too.
I'm still at a loss to understand why WinZip worked for me once, then not now, when I know I haven't changed anything. I'll have to look into WinZip closer, to see if something changed inadvertently.
@Peter, I didn't know anything about the "PK00" sequence. But everything else about the ePub files was the same with the older and latest sets, and the older sets opened fine. As another recent example, I have an ePub of "6 of One..."
on my site, which opens fine in DE, created with the same tools. (It's free, so feel free to check, compare and contrast.) How do I check the older epub documents to see which "PK" sequence is on them?
@Wallcraft, my earlier ePubs were broken up into individual HTML chapters. I didn't bother for the original test piece, but I believe the final test piece I posted was a Book Glutton ePub in one big file, shifted to .zip and back to .epub. (I've done so many tests, even I have to go back to make sure!) Anyway, any future novels will be in individual chapters.
@Kovid, I may still try to use Calibre, but I haven't gotten it to format properly yet... my initial attempts met with errors that I am still trying to work out (and which I may get back to you directly with questions, after I've exhausted the help file).