2.yom tov sheni - not all rabbonim pasken alike, and there is difference between bochurim/those who have families left behind in חו"ל/and those who arrive accompanied by families (it is my impression that this last example is where #1 plays the most role, ignoring דעתו לחזור)
I don't know that the Rebbe ever got involved in ruling about בן חו"ל who is in ארץ ישראל (though famously R' Mendel Futerfas "agreed" to assume a position in Kfar Chabad in some way that resulted in him always keeping 2 days Y"T IINM). However, there were definitely different indications from the Rebbe regarding בני ארץ-ישראל that came to be Y"T with the Rebbe at different times. With the משנה אחרונה seemingly being that they should observe Y"T just like the Rebbe - 2 days, though I believe בני ארץ-ישראל that kept 2 days when coming to the Rebbe were not given aliyos on 2nd day Y"T, or daven by the Omud.
4.the location of the dateline is a separate machlokes haposkim. the rebbe addressed it, but the מסקנא (in that letter) is hardly so clear-cut in some locations. (australia/NZ more clear, hawaii/alaska less so. fiji i really have no clue if anyone even asked). for some reason chabad rabbonim have concluded as you mentioned.
Where did the Rebbe address the location of the line, or come to a conclusion about it? In any event, there's definitely added complexity when talking about places that have changed their date at times in history. See:
https://www.bbc.com/news/world-asia-16351377 and
https://en.wikipedia.org/wiki/International_Date_Line#Historical_alterations which might present a problem even with the Philippines! There are definitely people (some shluchim to Australia) who were instructed to avoid crossing the IDL year-round.