Wow, that is quite some work JFincannon!
I have some more ideas....
Today's LROC Image of the Day
shows that you *can* get voids in an impact melt, if the cooling melt drains away leaving behind just a solidified chill crust. In this example, a small melt pond on the floor of Giordano Bruno crater drained away, causing the entire top to collapse, resulting in a spectacular jagged terrain (see my post on this elsewhere
). So, it is possible that this may occur in other places, too. Now, Copernicus crater is about 4 times the size of Giordano Bruno and has much more melt, so here you wouldn't get a small melt pond draining, but possibly parts of a large melt sheet draining? I don't know if this is possible, but it's an intriguing idea.
The reference you quote above is very interesting. I'm not sure if it applies in this situation. First, you don't have a regolith/megaregolith boundary, you have a very thin regolith over an impact melt sheet over the highland basement (not sure if you would call that megaregolith at this point?). So, I'm not sure where the gases would come from in this case. Secondly, the paper predicts features about 15 meters in diameter, while these ones are over 100 meters. Maybe the melt sheet affects the depth at which the gases collect, making that deeper, which results in a bigger cone? I have no idea. But very interesting to think about...
In any case, I agree that the round features you found are not craters. They are bigger and fresher than most of the craters in this area, but have no ejecta and seem to have a more spherical rather than conic morphology.
Truly interesting features. Thank you for finding these JFincannon!