1) If one is davening in a nussach sefard minyan during Pesach (even though he usually davens in a nussach ashkanaz shul (with no morid hatal in Chutz Laaretz)) when should he stop saying mashiv haruach? When the minyan stops after the public announcement in the sefard shul before mussaf or when the ashkenaz nussach starts which is only after mussaf?
2) If one by accidentally says the mashiv haruach umored hageshem when he is not supposed to can one cancel the words with a canceling thought? Do we say that just like toch kdai dibbur can save a bracha, so toch ddai cancelling thought can negate a dibbur? If not, is there any advice for this case? Or should one just repeat?
1) In this case, one should be noheg like the custom of the place where one is davening. As the Tur (114, citing from the Yerushalmi) explains, the idea of the pronouncement is that the tzibbur should all be davening the same davening, and it would not be right to change from the davening that is being said by the tzibbur. See also Mishnah Berurah 114:3.
2) No, a “cancelling thought” does not help to cancel words, because at the end of the day, the words have been said. For one who says “morid ha-tal,” the words “morid ha-tal,” when said within toch kedei dibbur, are able to “cancel” of supersede the previous words. For one who does not, the paragraph (from Atah Gibor) should be repeated.