Problem has been resolved - skip to the end for the punchline.
My previous method has been to download books (which open in ADE), copy to Calibre, and then transfer from Claibre to my PB. I have done this sucessfully with well over two dozen books, most of them DRMed, so I'm pretty certain I have the process right. Occasionally one or two showed the "unable to open file, may be damaged or protected" error, and after multilpe attempts I gave up on them, but that was fine because they were free anyway so I assumed something in the offer had expired. I also used this method with purchased books.
With this book, when I tried my usual method it seemed to work fine, went from ADE to Calibre to ereader okay, except when I trie dot open it on my PB it wouldn't open the file, "may be damaged or protected." I thoguht there must have been some glitch, perhaps I unplugged too soon, etc, so I tried it again. Same result. I made another download from my Kobo Library and tried again. Same result. I thought maybe I wasn't keeping my copies straight so deleted everything and tried again. Same result.
So then I decided to try skipping the Calibre step and going straight from ADE to PB. I hadn't don't this before, but it seemed pretty straightforward to drag and drop one of the books in the ADE library to the "pocket360" listed inside ADE under "bookshelves". That's when I got the note that (IIRC) said it cannot copy due to the DRM restrictions. At which point I started to lose it.
I tried a few more times and did actually manage to get the book onto my PB from ADE, but it was just like with Calibre, it would not let me open the file. Gave up, wrote frustrated post, went to bed.
Anyway, starting again today this time I got ADE's "Error#2038" (
this discussion details it) and so started fresh by deleting all copies and downloading again from my Kobo library.
This time it worked, file opens fine, I actually have my book!

Looks like whatever the error was has resolved itself.
Will still watch the video, though, to check and see if I missed anything. Thanks for the replies, everyone.