I think that with this sort of arrangement we must expect a level of failure, or non-completion, and not get too wound up by it. With but one exception I have been very pleased with the exchanges that I have been involved with through the APUG medium, both public and private. I agree that when someone doesn’t honour their commitment then they should be barred from further participation, and I think this ban should crossover to all the public exchanges, if possible. One PM reminder should be sufficient, whilst one good reason (as Bob outlines) for none completion should be accepted. I would expect anyone unable to complete their commitment to announce the fact either on the forum or by PM. After all circumstances do change, and sometimes very rapidly.
I must add my appreciation for the work that you put into arranging and monitoring this exchange Art.