Thanks for the detailed information, this is exactly what we need!
One of my characters which failed to transfer due to all my character slots being full on the destination server is now locked out of being able to be transferred. I can still log on to this character on my origin server, and I can still log on to my other successfully-transferred characters on my destination server, I just can't initiate the transfer processes for that previously-failed character.
I talked to the platform and web folks, and what's happening is that failed transfers are being interpreted as permanent failures that can't be retried. In the case of character counts, that's invalid, since you should be able to free up slots on the destination server or buy an additional character slot and retry the transfer successfully.
With the next code publish we're going to fix that behavior, but in the short term, we're going to start cleaning up failures occasionally so that you can retry the failed transfer. Just be sure to check that there is space available on the destination before you retry, or you'll get stuck again until the next cleanup! : )