Stuff you've learnt the hard way

[Owlie]

Did you get them off?

If you have enough clearance behind the bumper to pull the bolt out from the back, the easiest thing to do is drill off the heads.

Yes! WD-40 left overnight did the trick for all but one of them. The remaining one had a nut on it, and they had rusted to each other and to the little Y-shaped metal thing that holds the lower two bolts. We somehow pulled the plate off without undoing that bolt. My car is legal once again!