OK. I just went through that again. The exact steps were:
1. I selected a song in Total Commander and clicked play with PA. I got the message "too many retries". Probably because I tried that last night and then kept trying next track and previous track - to no avail. So,
2. I went back to PA settings and made sure both top level choices ("Internal Storage" and "SD Card") were ticked.
3. Then I did a full rescan. (That IS what I'd done before but this one took a lot longer). When it was done I had a bunch of folders and thousands of files selected. That makes sense since the card DOES have thousands of songs on it. However, nothing more than the two top level choices ("Internal Storage" and "SD Card") was still available.
4. I played songs through Total Commander again and was able to go to next track and previous track, but it WAS everything. Including ringtones and other stuff I just didn't want to hear. PA still showed nothing below those two top levels for library choices.
5. I shut down PA on my phone and went into another room to answer this on a computer. I just checked PA again. It still shows only those two choices top level choices ticked, but it's back to showing 0 folders and 0 tracks selected.
I figured it was probably a security/permissions thing last night and looked for anything I thought might have anything to do with PA or default apps, but couldn't find a thing.