Storms could have an effect on the Hellas's appearance, but you'll notice from Surveyor and MOLA data that Hellas is in effect a very deep and extensive depression that has been partially filled with what appear to be sediments, no doubt from some of the channels mentioned in Nick Hoffman's papers.

The problem I have is accounting for the extent and depth of the depression itself. So far as I know, there is no other impact structure of comparable size anywhere in the solar system which has not essentially rebounded to a level nearly that of the surrounding surface, nor is there a comparable feature on Mars that has not rebounded.

What geo-physical mechanism has allowed this one ancient basin on Mars, though partially filled with sediments, to remain an incredibly deep depression?

Why didn't rebound alone "refill" the Hellas basin?