Totally understand, been there on my scenic.
Renault use press fit captive nuts into blind sections of the chassis. The bolts rust in a little and the press fit no longer holds.... so the bolts turn and you get nowhere.
Various options I know off.
Grind off head, then knock in remnant and stick in a new captive nut (search on
, M6 I think) to replace was the one I went for. With lots of grease to stop them rusting.
Grind off head, leave remnant, drill alongside and use a self tapper to secure.. hex head screw is best for getting out again in the future.
Probably others will have other options.
One common reason to get the trays off is
change. I drilled a hole in my tray to give access. About 50mm dia.