Why not supported? If both the times Paul uses the construct ''Brother of the Lord'' he does so to distinguish apostles from mere baptized Christians, then surely the James of Gal 1:19 is not a leader or apostle (therefore he cannot be the James Pillar of Gal 2).
The question becomes, why would Paul use such a qualifier to distinguish between two different men, when both were respectively Apostles? James bar Zebedee was just as much an Apostle as James the Just, yet being he is singled out in Gal 1 as
brother of the Lord according to you and Carrier.
Again, the tradition that there were multiple James (James bar Zebedee, James the Just, James the Lesser, and James bar Alphaeus) came about after the Pauline letters, which only ever mention one, single James.
At any case, I can understand which is your interpretation. You think that the ''Lord'' was John. Do you mean just John the Baptist? Or a John son of Judas the Galilean ?
John the Baptist appears to be a character from tradition, and may be based on Theudas. It's possible, but not something I've bothered with too much. But, yes, I do think that John was the Lord.
I mean, is it just a coincidence that there are two figures that both have mothers named Mary, a brother named James, and are distinguished with a title that exalts them to divinity?
According to Georges Ory, the Christ descended on John the Baptist in the Gospel read by Celsus.
That's what the Manichaens believed as well.