Thanks everyone. I FINALLY figured it out by watching a gal's video https://www.adobe.com/designcen…..id=vid0078, which ended up being the lightbulb for me. I also had to make some educated guesses in areas that weren't clear to me.
i.e. in this gal's video, she was making headers based on a “style” she chose in Text Variables. Lightbulb!! I want back to my Text Variable, changed Character Style to Paragraph Style, and then choose my specific “Chapter Titles” style. I went back to the Master Page and selected the right side pages, and “Inserted” the Text Variable I created (Type > Text Variables > Insert)…and voila, all the headers appeared based on the Chapter Titles. I was both amazed and wanted to fall flat on the floor in shock and fear of “did I REALLY understand what I just achieved??” lol.
I did notice one problem: two of my chapters have a title that incorporates a colon in the middle. And the Variable put the SECOND HALF of the title as a header. That did not work. lol. So I've been manually changing those two chapter headers.
BOY THIS HAS ALL BEEN HARD. But, other than making sure the page numbers are correct in the Table of Contents, I'm happy to state that I've only got the Index left to do. Shudder shudder.