The only (minor) problem is that the GTK+ included is, I think, very out of date (the others don't appear to be so much).
But, I'm second-guessing my conclusion that GIMP's more current GTK+ installing over the older GTK+ is the source of the problem. I'd have to do some testing to see for sure, but I began to suspect last night that part of the problem was that I chose a poor test-case of a python add-in to verify whether python for GIMP was working. I discovered later last night that Sagenlicht's GURM, which I had been using as a quick test, requires some additional hacking in the .py and .ini files to make work... Still haven't gotten that one script working, but all the others I've downloaded so far are now working like a charm!
Thanks everybody for your patience with this guy who "knows just enough about computers to be dangerous, but not enough to be useful".