It's been a while since my original post and I've tried essentially every solution offered here. Still leaks from the same place: right rear corner and a little forward of that, sometimes running down over that little vent thingie on the bottom right. I remain suspicious the leak has something to do with either the luggage rack where it attaches in the rear, or that "gutter" that runs down the side covered by a rubber strip.
I've been leery of trying to remove the luggage rack end cap (due to concerns over possibly breaking it) and of removing the rubber "gutter" cover, for fear I'd damage it and have to have it professionally replaced...
If there are any new theories, or safe advice on how to do detective work on those two other possible sources, please share... This is making me (more) mental!