@Ernest BywaterFinally! Why couldn't anyone (online references, help sites or here) mention the the .opf file is named "content.opf". If I'd know that much, I could easily have found it, but no, they send me down a damn rabbit hole of .opf=.doc=deconstruct your ebook/you can't reconstruct a broken ePub file load of nonsense.
Plus, while I can now see how to muck with the .opf file, I originally started on an Amazon Author's forum discussion of opening Kindle files. So other than converting all of you Kindle documents into Calibre, deconstructing and reconstructing them and then re-converting it back into Kindle format. Somehow, I can't imagine that's how the mainstream publishers are doing it, now how Amazon itself is doing it.
Later Update: It turns out, you can access the content.opf file directly on the Mac, by going to the Calibre data files for the particular book, where it's stored as a separate file. However, the "reference type="start" still had no effect, and when I tried to unzip the files, it produced an unable nonsense file (I certainly didn't recognize it), so after downloading a NEW UnArchiver app, I was unable to decompress it, access the file, only to learn that it was the same as I'd edited from within Calibre itself.
I'd thought that Calibre was erasing it each time I generated a new file (testing out each), but actually, the "reference type="start"" command was simply being ignored each time.
So, unless anyone has any new ideas (like moving the "guide" commands to the top of the file (Calibre itself puts them at the end, which is where I added to it), I'm out of ideas. There doesn't seem to be any to adjusting the start position of eBooks, at least on Macs.