1. Why does the bottom half of a dissipating thunderstorm usually “disappear” before the top?

Answer 1

I'm just going to link you because I think it explains it in an easy enough way, and includes diagrams.
This is for dissipating ordinary tstorms, supercells dissipate in a slightly different way.

Answer 2

When a thunderstorm dissapates, the top does go first, since the moisture of the cloud has condensed into liquid and if falling as either rain or hail.