Hi CS,
Regarding your first point, modern-day Lubavitchers DID go off the rails. And it didn’t take five years, either. Most Lubavitchers still believe that he is Moshiach, which is bordering on apikorsus, as I’ve discussed at length in other threads. The ones that don’t believe this are afraid to speak up and correct this edited.
Regarding your second point, yes, Achdus Hashem is what it’s all about. The Arizal was the one who translated the kaballah of R. Shimon bar Yochai into an elucidated system, but he purposely clothed it in משלים. the Baal Shem Tov was מגלה that the whole purpose of Kaballah was to bring out the Achdus of Hashem in the world. All of our avodah, יחוד קב”ה ושכינתיה is based on this Achdus Hashem.
This was brought out by none other than one of the Baal Shem Tov’s big talmidim, R. Aharon Halevi Strusele, which the Mittler Rebbe ostracized. I heard from a Lubavitcher who was close to the Rebbe that The Rebbe wanted to reprint this sefer in addition to all of the other Chassidus he brought into publication, but was afraid of what the old-time Chabadniks would say. In any case, I strongly recommend that if you want to understand the entire system of Chabad the way that the Baal HaTanya himself envisioned it, that you read שערי היחוד ואמונה and שערי עבודה by R. Aharon Halevi. You will see that all of the Tefillah, especially Shacharis, is intended to be a form of התבוננות into יחוד השם. This was the vision of the Baal HaTanya, and it is THE fundamental יסוד of the original Lubavitch chassidus. Virtually every maamar that the Baal HaTanya gave discusses hisbonenus in some form.
In terms of my own learning, I own the maamarim of the Baal HaTanya, and the MIttler Rebbe, and the Tzemach Tzedek ( except the ones that are out of print). The latter two both knew the Baal HaTanya personally and their primary focus in all of those volumes of maamarim is to elucidate and explain the maamarim of the baal hatanya.
I am sure that the other, later Rebbes have also written excellent chassidus, up to and including the late Lubavitcher Rebbe, may his memory be a zechus for Klal Yisroel, but they are absolutely not required to understand the Baal HaTanya’s Torah.