Separate question - the case the Rosh gives on 47b of meruba bgadim and mashuach she'avar seems rather complex, and interprets she'avar on the replacement, whereas everywhere else it means the one being replaced.
Why doesn't the simple case work - Reuven is mashuach, then the shemen is nignaz, and Reuven develops a mum, and Shimon takes his place and is meruba b'gadim, so until Reuven heals only Shimon can do Avodah.
One last question: In the case the Rosh gives earlier, quoting R' Moshe (though I didn't find it in the Rambam when I peaked there, so likely a different R' Moshe), where the replacement is there for a long time period, and then the Kohein gadol becomes fit, is he saying they could both do avoda? Or just one?
Thanks for you help!
-Sender Klein
Sender Klein, New York, USA
1) Great question! It seems like there are some Rishonim who argue with the Rosh like you say. I will try to explain the Rosh's opinion.
The Rosh is looking for a case in which the Mashu'ach she'Avar is not really Bar Avodah anymore, and not a case that he is still waiting to heal and come back to be a Kohen Gadol. If you read the Rosh from the beginning, I think he says pretty clearly that even in a case where the Mashu'ach is still Tamei or still has a Mum, the Merubeh Begadim is first to be Nitma, and the Mashu'ach is first and more important because he regularly is Makriv the Par ha'Ba Al Kol ha'Mitzvos. This is why he is looking for a more complex case where he has a case of a Mashu'ach that is not meant to come back to be a Kohen Gadol at all.
2) As for your other great question: It seems like the Rosh is quoting Rabeinu Moshe me'Ivra (see also Perush ha'Rosh on Horiyos 13a, where the Rosh brings the same Perush in his name). The Rambam seems to argue with this opinion since he writes (in Hilchos Rotze'ach) that the reason why a Kohen Gadol who killed someone unintentionally is not permitted to go back to be a Kohen Gadol is that he was involved in such a terrible accident. It seems from this that if a Kohen Gadol was absent for a long term, but not involved in an accident of killing, he is welcome to go back to be the Kohen Gadol, unlike the opinion of Rabeinu Moshe me'Ivra.
It is pretty clear in the Rosh, Tosfos, and the Shitah Mekubetzes that the second one remains the main Kohen Gadol, and the first one has the status of a Kohen Gadol she'Avar, which means he does not go back to be a Kohen Hedyot because of the concept of Ma'alin ba'Kodesh v'Ein Moridin, but he does not serve with the eight garments of the Kohen Gadol because of the concern for discord ("Mishum Eivah").
There are Acharonim (the Ateres Rosh) who disagree and say that in a case where the first Kohen when on a long-term break, when he comes back the two Kohanim split the service equally. As I said, I think this is not the Pashtus in the way the Rishonim quote Rabeinu Moshe's opinion.
Kol Tuv,
Aharon Steiner