Maybe the first issue is “What is a Rebbe?” Maybe the first issue is “What is Torah?”
If you can grant that Torah is only what we can “prove” Hashem said, then a Rebbe has to fit that definition: Someone who helps you to know what Hashem says by means of “proving it”. I don’t mean to suggest that this is the only factor – there are many others – but perhaps the starting point is that Torah is not a “guessing game”.