Ah, DiapDealer, but I'm willing to learn from my mistakes.
Thanks to Jim's reply, I changed the version number to 2.0 in the content.opf, removed the toc.xhtml file from the OEBPS folder, and deleted the reference to that file from the content.opf. Validated fine as 2.0.
I appreciate the hand-holding. Sometimes it's exactly what I need. And I'm always humbled by the kindness and patience shown by the members here.