Interesting idea you have about fastening them. I'll have to look into this.Wish you showed a pic of them removed before cleaning. I'd expect crap to get behind them, it's inventible.
Interesting those plastic rivets are so hard to get out.
One simple Idea. Put OEM Rivets back in (or after market).
Drill Pilot hole in the Rivet plug, off center.
Then use a Screw and washer into the Rivet plug. If it's offset center enough it should not rotate when removing.
One problem with this is, is it opens the gap between flap and the well liner allowing more crap to collect behind it.
What you could do is punch out the hole in the flap to the size of the rivet flange. That would make flap flush to well liner again. Then use a really large washer
This is all reversible and who cares about a $30 set of flaps and some rivets.
Or find better reusable rivets, which might be hard to do. So much Chinese crap out there.
I just installed similar flaps on my Model X. They only use one rivet. But what's nice on the Model X is, it's all Plastic on Plastic. And only front flaps are really needed. But I suspect they are the same crappy rivets.
I'm tempted to drill all new holes (in both the flaps and the wheel liner) and not even use the fasteners. The holes would be so small, and it would be so easy to just unscrew them as needed.
Then I'd just need to find a way to use weather stripping or something to prevent the gunk from getting behind it.