@RSO,
“I asked for a case where this did not involve a public relations coup, as was the case with the son of an Israeli rebbe.”
Very well. There’s a very well known and very large family or Lubavitchers, I won’t name them here, but anyone intimately familiar with Chabad will know whom I referring to. They’re spread out over Canada and American, and have never changed their Levush at all, at the Rebbes request. (Incidentally, their father/ grandfather wrote Rav Moshe Feinsteins Rabbeinu Tams tefilin.)
Additionally, in the late 70’s and early 80’s there was a group of Satmer Chassidim who became Lubavitch. (Together with their Rosh Yeshiva) they too didn’t change their Levush.
Now, you need to differentiate between Minhag and Halachic Shita. Levush is (at least in Chabad) considered a Minhag, and changing it is discouraged. Tefilin on the other hand, and how they’re wrapped and their size is Halachic Shita. So if someone chooses to become a Chabad Chossid, he would want to keep the Piskei Dinim of our Poskim, namely the Shulchan Aruch Harav and the Tzemach Tzedek, (as well as Halachic rulings of the other Rebbeim).
I don’t see why a sefardi Lubavitcher wouldn’t eat rice on Pesach as it’s an Ashkenazi / Sefardi thing, not specific to Chabad. I’d assume that those who don’t would like to be like everyone else in their community, and I don’t see why you’d have an issue with that.