I’m not quite sure I follow your logic. You have 2 ossuaries out of hundreds from this time that demonstrate the rarity of a brothers inscription being included. A fairly liberal statistic of perhaps 20 other James, son of Joseph, brother of Jesus in this region during this time period. The statistical odds that even one of these James ossuaries would have a brother inclusion in the inscription is remote to begin with, but if we have one, we have to pick the Jesus that was of greatest importance when compared to the other Jesus’ whom also could have been included. This natural selection leads us to Jesus of Nazareth as the person with greatest attributable probability. I think you are falsely attributing the common popularity of the names to the uncommon and unique combination of names on this ossuary when including a link to a brother of importance named Jesus. The fairly ubiquitous opinion among the experts and scholars is that if this James ossuary turns out to be real, then it could easily be the genuine ossuary of James the Just, brother of Jesus.
I’m confused on your mention of the Protoevangelium of James. You seem to bolster it’s historical significance and reliability, yet you seem to be inclined to believe a “cousin” theory. As I mentioned earlier, the Protoevangelium of James explains Jesus’ brothers and sisters, James included, as being children from a previous marriage, making James an older step brother. This was the popular Catholic opinion until St. Jerome defended the cousin theory in the 4th century.
My only real point is that all of this is ultimately meaningless. Since none of it affects the deposit of Faith, and none of it can be relied upon to found a historical portrait, it remains a mere curiousity. A single disputed ossuary does not make for any groundbreaking revelations, especially in terms of history. If we had any surrounding testimony about the death of James and what was done with his body which coincided with the ossuary, we’d have something of greater historical value, but as it stands all we have is a box for bones with some names on it.
Even if it were the historical ossuary of James, son of Joseph, brother of Christ, it wouldn’t point to him being the son of Joseph the husband of Mary, since the ambiguous term for brother/cousin is still used, and since that term can be used for any extended kin of the same generation, the ratio of probable matches goes up considerably; if he was related by blood in any way to Jesus, and of the same basic generation, it would be worthy of note. It could be Joseph, brother of Mary, or Joseph, cousin of Joseph the step-Father of Christ, or what have you.
Even if we conclude that it is exactly as you say, the step-brother of Christ, it amounts to nothing. No matter of Faith rests on Joseph not having other, prior children, and in fact it’s quite likely given the testimony of the Early Church.
So all this adds up to is that maybe we have a box of bones that might be related in some way to Christ, and might even be the step-brother of Christ. Not exactly a solid find, no matter how you cut it.
I’m not invested in any explaination, since none of the likely ones have any bearing on history or on Faith. The possibilities aren’t strong enough to make any historical judgements, and they don’t even come close to touching on matters of Faith. So whether the term means brother or cousin in this case is irrelevant to me; the box remains an uncontroversial box of bones.
Peace and God bless!