I have no other software monitoring folders, but just to rule out the tethering software (CamFi) as the culprit, I shut it off and simulated doing a tethered shoot by using Finder to manually copy an NEF into the source folder (while PM's Live Ingest was running) so I could see if I still wound up with the tiny JPG and the XMP files in the destination. And while the XMP file IS still created (and I wish there was a way not to), the JPG is NOT created, so apparently the tethering software, for some bizarre reason, is doing this. Which is really weird, but not apparently anything to do with PM, so thanks for your help and sorry for the waste of time.