Leading half the prayers

Question

Assuming one is to follow the congregation's nusach when leading the prayers and one's own otherwise,

• what should he do if he's asked to lead the prayers from "Shochen ad", and agrees, before the start of p'suke d'zimra? Should he follow the congregation's nusach for p'suke d'zimra also, or is that not necessary?

And if the answer to that is that he should follow the congregation's nusach, then

• what about if he's asked during "Az yashir"? Should he refuse to pray on the grounds that he's in the middle of shacharis in the wrong nusach, or agree and switch?

If the latter (agree and switch),

• should he switch only for "Shochen ad" or also for "Nishmas"?

Sources if possible and reasoning otherwise, please.

Update: Another, related question: What if someone leads the t'fila until p'suke d'zimra starts or until "Shochen ad" and then stands down: does he need to continue in the congregation's nusach?

Answer 1

Rabbi Moshe Feinstein's guidance on what to do if your personal text differs from the congregation's (and you're just sitting in your seat, not the chazan) is to say your text for anything that's private and quiet, and the congregation's for anything that's public and out-loud.

Rav Moshe then discusses which exact parts of Kedusha fall into which of those categories; while I don't recall his exact conclusion, it appears you would wind up saying a hybrid kedusha that day -- and that's not the end of the world.

Extrapolating from that, I would argue:

• P'sukei d'zimra is inherently a private prayer. Some places (including Chaim Berlin yeshiva to this day, from what I hear) don't even have a chazan up there for it; those that do, do so to help with pacing or decorum, not that it's required. So you're free to follow whatever text you like.
• If you wind up saying a hybrid text because of a personal/congregational conflict, that's okay. If you can switch back-and-forth in the lines of Kedusha itself, it should be fine to say all of p'sukei d'zimra with your text, and then switch to the congregation's text at whatever point it becomes out-loud -- in this case, that would be when you stand up as chazan at Shochen Ad.