Hmmm.... Interesting points....

"1) The floor *has* rebounded."

Well... the issue here is that Hellas is supposedly so very old, formed when the Mars was a mere pup, and yet it is still an extremely deep hole, even though partly filled with sediments. Ought it not to have rebounded substantially more than it has? Rather like similar basins (Argyre and Isidis on Mars, Caloris on Mercury, Mare Orientale on the Moon, among others)? Given that the Martian lithosphere was "only 10 km thick- !!" at the time of Hellas' formation, rebound should have all but erased its depth. But it didn't. Why not? (Do you really expect to form a 60km deep hole in a fluid mantle?)

2) There is ample evidence for (some) sediment and lava infill of the basin. Question - how much is "some"?

And... ? It looks like the sedimentary depth in Hellas is considerable (several kilometers at minimum). Some has no doubt come from the channels and gullies around the rim... Some of this sediment appears to have eroded substantially... But erosion by itself cannot account for the depth of the basin.

I just wonder if there is some internal mechanism that could be partly or largely responsible for the uniqueness of the Hellas Basin on Mars.