"That is certainly an unprovable argument, I hope you realize that."
If one considers the Scriptures not to be accurately recounting various events, then yes, it is unprovable. I wasn't intending to prove anything, but simply pointing out that your statement that St. John the Baptist would rightly have been locked up for madness had no-one else seen the dove or heard the voice is an opinion that is entirely your own, and that is not supported by the approach of the Orthodox Church's spiritual life as it is recorded for us.
"It also opens Pandora's Box for every deranged mind to claim divine visions and voices, as some sort of divinely inspired gnosis."
If one doesn't trust that the Church is capable of discerning between delusion and truth, that is true.
FWIW I'm sure that John The Baptist didn't need to hear voices or see visions for the casual observer to think he was a hamburger or two short of a happy meal. The guy ate grasshoppers and yelled "Repent!!!" at everyone who walked by.
Counting all the heresies that came form the east, one must never doubt that even the Church is capable of discerning delusions as truth.