Dear RSo,
Ben acher ben is my assumption. Could be it’s wrong. That he is ben acher ben was a guess. Also, could be wrong. Just for fun, I advocate that he is qualified until you prove otherwise.
I have heard personally about his lomdus from old Lakewooders that went to him.
I never heard them mention a Viznitz Rebbe. Who do you mean, Reb Mordechai rom Monsey? He was much younger almost the same age as the American Talmidim. The rest were in EY. As to the other two Great Rebbes I explicitly heard they only went to them for reasons unrelated to the Limud Hatorah aspect. In the old days the Yeshivishe Velt was able to compartmentalize their attitudes toward other yidden. Nobody was black and white to them.