The distaff side of the Chabad dynasty

The passing of Barry Gourary, nephew of the the 7th Lubavitcher Rebbe (M.M. Schneersohn z'l) and the only grandson of the 6th Lubavitcher Rebbe z'l (Y.Y. Schneersohn z'l), is a time to reflect on the dynastic struggles in Chabad. As Tsemach Atlas, Shmarya and others relate, Barry could have been the seventh Rebbe (see Tsemach's 'Update #2' on his posting for a series of links). Instead, as result of feuding at the time of the passing of the 6th Rebbe, he and his mother were permanently sidelined out of the Chabad court. He entered public profile again at the time of the lawsuit over the Rebbe's library - where the Judge, if I remember correctly, was Reinhold Niebuhr's daughter's former husband, and Rabbi Louis Jacobs, long anathemized by the Orthodox establishment in the UK, appeared as an expert witness on Hasidic lore for Chabad. (Seeking to discredit him, Barry's attorney asked him if it was true that he had been rejected by Britain's then Chief Rabbi as the 'Head of Jews College in Oxford' on grounds of theological unsuitability. Rabbi Jacobs was able to answer in the negative and avoid the challenge to his credibility - not because the relationship with the Chief Rabbi had been incorrectly described, but because Jews' College was in London!) Anyway, the point of this posting is to ask for some info: Years ago, I was told by an impeccable source that the reason why the 'Mittler Rebbe' was so called was not because he came betweeen the first and third Rebbes, but because he was the middle son of the Baal Hatanya, and the eldest son had become a Russian Orthodox priest. At one time I saw a photo that was apparently of the apostate son's gravestone in a Russian Orthodox cemetery. I have only ever seen sparse references to this son in print. Can anyone give comprehensive information??? References ????