For now, if I can cite the scholarship on this 'probability' point from Stanley E. Porter: "the criterion of embarrassment argues that material that cannot be readily explained as created by the early church has a high probability of authenticity...these include the baptism of Jesus by John the Baptist, where the event seems to place Jesus in a subordinate position to John" (Porter, Criteria for Authenticity 77-79).
Fair enough, but the criterion of embarrassment seems to apply only after Jesus becomes marketed as the living God. If the founders tale preceded the finalization of this theology, it might very well have found its way into gMk as a selling point and subsequently, from Meier's 1999 The Present State of the 'Third Quest' ...
However, the gospel sources betray an increasing uneasiness or embarrassment with the superior, sinless Jesus being baptized with a baptism of repentance for the forgiveness of sins by his supposed inferior, John the Baptist.
And Meier goes on to caution:
While embarrassment, as a distinct criterion, has its own force and value, it also has, like the other criteria, its own built-in limitations. First, relatively little material in the gospels falls under this criterion. Second, there is the hermeneutical problem that what we might judge embarrassing today might not seem embarrassing for the first Christian Jews.
I am way out of my league here but I suspect that any claim beyond "not implausible" is to pretend to know more than is known.