Popes may well have the power to lift excommunications posthumously. I am predicting that they will not in the case of dead Marcel or dead anyone else in that schism other than, perhaps, Castro de Meyer. Why on earth would a pope lift such a meritorious excommunication?
It doesn't matter if the famous "excommunications" are lifted by this pope or future popes. They are legal harassments only and are otherwise null and void. They may therefore be ignored by good men--as they are.