What "kind" of force (s) would "grab" the V/S, ripping it of without touching the rudder?
Inertial forces, but the afterbody failed around the vertical, i.e., the VS didn't depart the afterbody, the afterbody departed the vertical.
Assuming the report is factual, it could easily have been backed up with reversion of debris to find ground zero, and demonstration of each item showing signs of downward compression force along the length of the fuselage. That would be convincing analysis, yet it is missing in the report. Won't speculate on what that omission means though.
If they hit wings level, could imply they had attitude. If they had attitude, they could have gotten out of a stall if in one. Flat spin is another matter. Said impact heading was in direction of flight, though don't know how they could know that. Downdraft from the thunderstorm could partially explain crash location proximity to last reported position.
Not blaming this on the pilots, keeping an open mind, without prejudice.