Thanks all. Yes, I saw rivnut installers at a DIY auto store, but these do not look like “typical” rivnuts. A 5mm fastener bottoms out after only a few turns, maybe 4-5mm deep. I can see a narrower sleeve inside the “rivnut” that stops a fastener from going all the way down/through.
Anyway, masking tape around the two that were loose and worked JB Weld epoxy under them and that seems to work. I just will not tighten too much when I attach bottle cages! Maybe some blue loctite thread locker to keep from vibrating loose? Or lock washers may be better…..
-Robin