No, I am not going to question his authority, just pointing out that the churches must have known John intimately so that when his letter circulated, even with "servant" rather than "apostle" or "Elder", they knew who he was. With that reputation among the churches, you would have thought Paul would have referenced him in his letters, like he did just about every one else.
I still must be missing something. Early date or late date, Paul was dead by the time the vision of Revelation was given. We have no evidence one way or another that John was even associated with the seven churches back in the late 50s/early 60s. Besides, Paul was able to cover the same territory in a rather short period of time in his first missionary journey. What makes you think John could not do the same?
So I'm really at a loss to figure out the point you're trying to make. But it's fun to speculate.