I think all these ideas about using prussics and stuff to effect a transfer are actually quite misguided.
Simple is better, and simple is safer IMO. What I do in a case like this is simply to use two belay devices. Lower until the knot is jammed on the first device, then get climber to unweight rope momentarily while I grab the other rope, and pop the 1st device off. Then climber weights rope again, and lower away.
With communication and preplanning this is a very simple and safe system.
I bet that a lot of people don't even know the proper way to set up a weight transfer with tied off munters and all that jazz. Not to mention that it could turn into a bit of a clusterf%^&k on your harness.
As to the accident, I have really no idea what happened, not having been there, and so have no comments on what went wrong.