Why are Paul's writings in the Bible?

[GrandizerII]

Paul, in his undisputed letters, says he met this same fellow's brother.  And the existence of that brother is supported by Josephus.  Then later the proto-orthodox tradition that became pre-eminent in the Jesus movement developed a tradition that Mary was ever-virgin, so they attempted, Stalin-like, to erase James from history.  But today mythicists tell us the same adherents of the proto-orthodox view added interpolations mentioning "James the brother of the Lord" to every extant copy of Galatians and Antiquities of the Jews.  That takes us into grand conspiracy territory, and the same impulse that led to my atheism makes me critical of mythicism.

If you want to know what mythicists tell us about TF2 it is that Paul did mention James brother of the Lord, but that seemed to be a title as he ascribes to other characters to be the brother of the Lord as well. Josephus did mention James brother of Jesus, but in the continuation says it is "Jesus, the son of  Damneus." The fragment “who was called Christ” was inserted into the text very clumsily since it is different Jesus, and a jew, Josephus, would not call someone messiah (Christ).

Ok, I'll comment only on what Paul said here, leaving Josephus for another time:

This is good, FM, because this is specific enough for us to analyse and see where we can go with this. Now, as I've stated more than once before, I'm all about parsimony here (and also about what scholars themselves say, but we can put that aside). For me, parsimony isn't just simply simplicity perse. It's about what is the "cleanest" account here, given what we can observe in the texts.

If we look at where Paul talks about the brothers of the Lord (in 1 Corinthians), he makes a distinction between them on one hand and Peter, Barnabas, and himself (and also Apostles in general) on the other hand. If "brothers of the Lord" is meant to be a title referring to a specific subgroup of Christ-believers, rather than literal siblings of Jesus, then what passage/document can we refer to to support this interpretation? Without that support, we probably don't have any good grounds to reject the literal interpretation that coheres with a passage in another early and plausibly independent source (independent from Paul) that lists James as one of Jesus' brothers (i.e., Mark). And there is something about the special status of a subgroup of Christ-believers that demands some support for it. And even if there was some remote/obscure support for the idea that there was indeed a special subgroup of Christ-believers called "brothers of the Lord" that had nothing to do with a flesh-and-blood relationship with the Lord, then does it lead to a "cleaner" account? Or is it rather "ad-hocy" relative to this historicist Jesus account?