Time, when Rav Charlap first came to Rav Kook’s yeshiva he addressed him in creole German. Rav Kook said “Being that you are new here I will not hold it against you. However, you should know that one does not address me in Jargon”. “Is there a mitzva to speak Hebrew ” by Rabbi ephraim Sprecher (online) where he quotes various sources, including RMF, as saying that there is such a mitzva. The Sheeta Mekubatzet even quotes the Raavad as saying that one might not fulfill the mitzva of talmud Torah in another language.
As for Rav Reuven’s alleged speech, it would certainly surprise Sephardic and Yemenite Jews (as would being called “Yidden”).
Shabbatai Tzvi did not institute saying Birkat Cohanim in Chul. This is the position of the Mechaber. The Gra and Rav Chaim of Volozhin also wanted to institute it but were unsuccessful.
