Would a 50 mph aircraft holding a ground pattern in a 20 mph cross wind risk stalling by abruptly rolling away from a crosswind?
Your question is flawed because it is creating a link between two different things.  Let's deal with them one at a time:

In the title question you ask whether a plane holding a crosswind correction in a landing pattern risks stalling by rolling abruptly away from a crosswind.  Ignoring for a minute that doing anything "abruptly" imposes some additional level of risk, (depending of course on the severity) the answer is no.  Turning into, or away from the wind in a "uniformly moving airmass" does not alter the airspeed of the aircraft.  You said as much yourself.  When ready for downwind you simply turn to a new heading for that leg as you would normally.  (in this case 090 because you will now have a tailwind.)

The very concept of "wind" only matters when you consider the surface of the earth.  The airplane doesn't recognize or care what the ground is doing, only the airmass matters.

Now to the second part; would flying near stall speed and then abruptly rolling and yawing the aircraft risk a stall?  Yes, absolutely yes!  As well as a spin...  Those are the main two ingredients of a spin, and every pilot is taught to avoid them, especially down low.  Statistically this is probably one of the main GA pilot killers.

So, let's further unlink the two concepts.  You seem to be espousing an unbalanced slip approach to maintaining heading and ground track in the crosswind turn, furthermore, doing it right around stall speed!  (Sorry, that wasn't you, it was pilot Bob...) This is improper and unsafe technique.  You should not attempt to maintain aircraft heading of 180 on the crosswind, you should fly 190, or 195, or whatever heading will allow you to maintain a consistent ground track.  You should do it wings level, in balanced flight, at a safe and appropriate airspeed for your aircraft in the pattern.  The same airspeed you fly when the airmass is not moving relative to the ground.  This wind correction technique is called "crabbing".

Bottom line, it isn't the wind creating any additional risk, it is pilot Bob's improper technique.