The gemorah learns from aviv that a cohen is metamie to a meis mitzvah. and from eimo a different drasha. But rashi in chumash Vayikrah 21:11 mentions both aviv and eimo as the source of the drashaa that a cohen is metamie. This is not a precise language as is the way of rashi . Only Rabbi Akiva saya that both words are for one drashs. But we dont follow his ruling here?
Rebbi Akiva uses "Aviv v'Imo" of Nazir to teach that he may be Metamei for a Mes Mitzvah. He does not use "Aviv v'Imo" of Kohen Gadol for this purpose. This only makes your question more difficult, as the Rashi which you quote is commenting on the words in the Parshah of Kohen Gadol.
However, Rashi is citing the words of the Beraisa (and the Toras Kohanim) which says that the words "l'Aviv u'l'Imo" are permitting him to be Metamei for a Mes Mitzvah. Only when the Gemara asks "l'Imo Lamah Li" does the Gemara change and say that "l'Imo" is Mufneh for the Gezeirah Shavah to teach us that the Kohen Godel may be Metamei for "Zivasam" and "Nigasam."
Since the Limud is not actually from the word "l'Imo," but rather we say that "l'Imo" is Mufneh, the words of the Beraisa remain true -- that the Limud of Mes Mitzvah is from the whole phrase ("l'Aviv u'l'Imo"). It is jsut that since we could just as well have learned it from "Aviv" alone, "Imo" remains Mufneh for the Gezeirah Shavah. However, if we had learned a different Limud from the word "l'Imo" itself, then it would be a Stirah to the original words of the Beraisa ("l'Aviv u'l'Imo Lo Yitam'u Aval Mitama l'Meis
Mitzvah").
Dov