To Borlani (and anyone else who might help):

I have made my way through the thickets, and, using Sigil, am at the very last step for having a valid epub.

My problem: a small number of errors that need fixed. Is there a resource to help with this (free or low cost?)

Here are the errors:
ID value 'heading_id_2' is not unique
This resource is reachable but not present in the OPF <manifest>. "Reachable" means that a reference of some kind that points to this resource exists in the epub.

Only one "reachable" error, a number of "not unique" errors.

Wraylewis