Fixed my own problem FTW
I had to start over from scratch with my PE due to a stalling-while-booting issue, so I got to do this all over again, and was having crap luck at it. Finally decided to reread the entire thread and not just the content-heavy first couple of pages, and lo and behold I see that I had actually posted in it the last time I did this. Yeah, you guessed it, the thing I said I had to do the first time around? Finally made it work this time, too.
So, for anyone else who's tried the lengthier and/or repeated suggestions and still had no luck? Format the CRAP out of that sucker with SDFormatter -- specifically, use the Full (Overwrite) format, not the Quick format -- and then go into Minitool to add the second partition (i.e. resize the original partition to a smaller size to make room for a second one, then create the second one). Both times -- today and when I did it the first time over a year ago -- that was the trick that sweet-talked my PE into recognizing the SD card as usable. Everything else worked like a charm.