[Harpkatt]

I used rivet nuts to install my handmade metal floor pan beneath my roadster. works perfectly and I've had it in and out multiple times with no issue. I've been dealing with rivnuts for a long time as I was a bicycle tech for a long time, they are used to hold water bottle cages to the frames. The main issues I've had were the fastener seizing into the rivnut . Corrosion from perspiration will lock the two pieces together then when loosening cause them both to spin. Antiseize, grease or wax goes a long way here.

That said I'd say there isn't too much to worry about using rivnuts. if there's an issue file the head down or drill and they pop right out. In a bike frame they are captive so there its a problem there with removing from the tubing. If there's clearance behind it for it to come out it's not an issue and I wouldn't overthink it.

EDIT I guess I missed that you intend to use these to hold the seat to the floor, not the floor to the frame. I would say rivnuts are probably not strong enough for this application.