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?