David
OK, but taking what you wrote at face value, I asked you a few questions and pointed out some difficulties with what you reported.
I do this review in the next paragraph and conclude it is not well attested.
Well, it is tricky to report your own view along with a survey of opposing views all in one dish. The view you attribute to others is not supported by the facts, as used in the reasons
you cite to explain
their conclusion. This raises a suspicion that your best-equipped opponents don't actually argue that way.
It is a problem with the "HJ scholarly consensus" that it comprises a mixed bag of "scholars." It is possible that some of them do advance the view you report for the reasons you cite. But if the reason is faulty, then one of us needs to point that out, and a search should begin for why other scholars, who don't make these mistakes, nevertheless draw the same conclusion (if they do).
I am not aware that Josephus disallows the evangelical version about John. Another different thing is that Jewish ritual ablutions had other aims.
Of course, Josephus doesn't comment on Christian thinking about baptism. We know that the idea of sacramental efficacy (that the Christian baptism ritual mitigates previous sins) is prevalent in the first few centuries of the Christian Era, long before any "evangelicals" show up. Jews do indeed wash for a variety of reasons, but the Christian movement was a largely Gentile affair even when Josephus was writing, and has not much looked back to its roots since then.
For Josephus on the Dunker, see
Antiquities XVIII 5.2, which includes
{John} was a good man, and commanded the Jews to exercise virtue, both as to righteousness towards one another, and piety towards God, and so to come to baptism; for that the washing [with water] would be acceptable to him, if they made use of it, not in order to the putting away [or the remission] of some sins [only], but for the purification of the body; supposing still that the soul was thoroughly purified beforehand by righteousness.
http://www.ccel.org/j/josephus/works/ant-18.htm
Whether this contradicts or expands upon
Mark 1:4 is debatable ("John [the] Baptist appeared in the desert proclaiming a baptism of repentance for the forgiveness of sins."). It is interesting that "Mark" has John crisply distinguish his baptism from the coming Christ's at 1:8 ("...I have baptized you with water; he will baptize you with the holy Spirit.").
On the
narrow issue, then, there is ample textual support, secular and canonical, to deny an imputation of sinfulness to anybody, including Jesus, whom John may have dunked. It is, according to Josephus' telling, in black letters, an affirmation of achieved righteousness.