Well, I don’t know about rehabilitating him, but I think it is possible to think you were properly elected Pope when you weren’t. Peter de Luna (as I always heard him called) may well have been acting in good faith. St. Vincent Ferrer backed him as well. (St. Vincent had incredible charisms, including raising people from the dead, but apparently he didn’t have the gift of discernment).
One could say the same about Bergoglio and his own "election".
St. Gallen???
St Vincent Ferrer was from Valencia as well...so he was definitely predisposed to defend a fellow Catalan!
However, the subject of the Avignon popes is so complicated that it is quite possible that he was acting in good faith (he was pope for a fair number of years).
An anti-pope is not the same as the “anti-Christ;” it is simply somebody who claims to be or is even approved to be the reigning pope where there is somebody else already installed. In other words, they are not necessarily heretics.
In our current case, however...