Well, I am not exactly sure what happened, but I basically started over:
- deleted the custom table from the database
- exported the metabox
- unpublished the non-working metabox
- imported the metabox (I got an error here: "Cannot load meta-box-import." but I clicked "back" and the import actually succeeded, so I continued.)
- added test data to a page
- ALL saved successfully...
Weird, but I'm happy! If you want to know more details just reach out.