Some time later... I think I've figured it out. After putzing around a bit further, I discovered the repair worked, for the most part. The only pix that I can't seem to export or edit in the Photos app are the ones I also have over in Elements (makes sense). I just exported some of those to my desktop and imported them back into Photos and now I'm able to edit and (re)export them.
Geez! If Apple hadn't messed with it's perfectly good batch change feature when they did the big overhaul from iPhoto to Photos, I wouldn't have purchased PSE and probably wouldn't have had all this hassle. I'll export/import the pix I have (just this year's to date) in Elements back into Photos and only use Elements as a more in-depth editor.
Thank you so much, Leonie, for your help and patience!