AAQ – chasidim use the term sefard because they’re basing it on mekubalim who were almost completely sefardi. Chabad calls its nusach Ari for the same reason.
Mesivta; you might want to ask a shailoh about what the correct course of action is for you. Davening Ashkenaz for the duration of your time in that yeshiva is something many rabbonim would encourage.