Without any insinuations about who is or is not ‘a gadol’, it is abundantly clear that while one cannot be great without being great in Torah, that alone does not serve as a determinant. There are countless examples of great people who were subservient to their Rebbeim, whom they considered to be great, despite a superiority in Torah knowledge and/or learning abilities.
I’m not going so much for the system of appointment by earlier recognized authority. I think, to use the previous analogy to the medical field, that you don’t need to be an expert to determine who the top guys are, just sufficiently knowledgeable. So a general consensus by ‘regular’ Rabbanaim/leaders would establish for me a guidelines as to who’s authority might be overriding.