That is what I would vote for. I don't think we are going to come up with anything new. And for the comments about weak antigens, or not: Though they might not be weak, (and I agree we don't see too many either), I still think that could indicate a subgroup. who knows it might have just been a fluke!
And, remember, if the baby is AB, the A might be an A2 or other subgroup, which is more common in AB than A.