> Instead, the computer models showed that after a giant impact, the dense rocky material would quickly settle back down

At planetary scales, never underestimate the power of gravity to force everything to settle.

In particular, I'm thinking of soft sci-fi settings where a planet or moon is artistically unspherical, either from fantastic geologic processes, or because it was blasted last episode with the hyper-mega-beam.

Nope, that's like trying to scoop a permanent hole out of a mountain of custard.