Is one to recite Nakdishach if he is Davening by a congregation that says Nikadesh?[1]

If one is Davening by an Ashkenazi Minyan who recites Nekadesh instead of Nakdishach, he is to recite the Nussach of the congregation, which is Nikadesh.[2] The same applies during Musaf, that he is to recite Nakdishach in place of Keser. If however there is a Minyan of people saying the Nussach of Nakdishach/Keser, then he may recite it.[3]

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[1] Igros Moshe 2/23; Minchas Yitzchak 7/5; See Piskeiy Teshuvos 68 footnote 29; See however Maharam Brisk 2/28; Piskeiy Dinim Tzemach Tzedek 236 regarding that Davening a different Nussach than the congregation does not involve Lo Sisgodedu [He however does not discuss the scenario of Nakdishach] Maaseh Rav of Rav Nassan Adler that Davened Nussach Arizal in his congregation of Nusach Ashkenaz, brought in Chasam Sofer O.C. 16; Sefer Haminhagim p. 5 [English] footnote 16; Igros Kodesh 10/208; 14/391 19/4 [Published in Shulchan Menachem 1/171-174]

[2] The reason: As even if there is no issue of Lo Sisgodedu, one requires a Minyan when saying Nakdishach, and if the congregation is saying a different Nussach then he does not have a Minyan for his recital. [ibid; See Admur 125/1]

[3] The reason: As the entire issue with saying Nakdishach is that he cannot say a Davar Shebekudhsa without a Minyan. Thus, if there are at least ten people saying it, then he has a Minyan available. Thus, by a mixed Minyan in which the Sefaradim and Chassidim all say Nakdishach, one may also do so even though the Chazan Davens Nussach Ashkenaz.