This definitely is a delicate situation. Frankly, I would not attempt to pass the pedestrians; that would be bad form. However, this is what I would do in regards to the 20 mph cyclist, who really ought to know better than to be riding at those speeds on a MUP.
1) Come to an abrupt stop.
2) Dismount and park your bike across the entire width of the MUP, effectively blocking further egress.
3) As the other cyclist approaches, put both hands up in the air as in a stop signal.
4) As you raise your hands, yell, "HALT, CRIMINAL!" at the top of your lungs. Stare intensely straight ahead as you do this - do not make eye contact with the other cyclist.
The category 6 speed demon will have no choice but to come to a complete stop in front of you, and a collision between cyclist and pedestrian will have been narrowly avoided.