I think the Doherty's point of view on this subject is a relevant one from the mythicism. You can see it here:
http://vridar.org/2010/05/02/applying-sound-historical-methodology-to-james-the-brother-of-the-lord/
I avoid linguistic considerations because I am not an expert in this specialized field.
Regarding the other non technical arguments I think they are very disputable. But in general, the argument that the expression "brother of the Lord” differentiates contextually James to the others mentioned along him seems consistent. It discards the use of "brother" in a more general sense as “disciple” or similar. Doherty's suppositions in order to weaken this implication seem to me more inconsistent. There is no reasonable motive to suppose that John or Peter don't were called "brother" because they would be familiar with the Galatians or so. Yes, it introduces the shadow of a doubt, but sustained by nothing it is a weak objection.
Either Paul wants to say that James was the blood brother of Jesus or someone very close to him. In both cases the supposition that he was a source of information about Jesus seems coherent.
But if James or any other close person informed Paul about Jesus in his journeys to Jerusalem it must be said they were very bad "informers" because Paul tells practically nothing in his letters about the life or sayings of Jesus. Moreover, his stories about Jesus' appearances post mortem are incredible or disagree with the other evangelical sources.
If I was a Christian I would get a big headache with Paul accounts of their own adventures. I would be specially reluctant to admit their accounts without seeing the opposite point of view. But to admit he says that he saw the brother of Jesus in Jerusalem, why not? Another different thing is to know what they said each other.