DaMoshe:
I think R’ Chaim Berlin would agree that b’etzem nusach sefard is not the proper nusach. His point was that once the shul is davening a certain nusach, you can’t publicly deviate. R’ Aharon managed to change the tzibur’s nusach, but if he hadn’t been able to secure the amud, he might have davened nusach sefard (for the public parts).