The 'Why it Works' is wonderfully done and I don't think it could be better without being a whole lot longer.

I think the reason there is virtually no Doumeki/Himawari in fandom is because the two of them are so very focused on Watanuki and his protection. Their relationship wouldn't be so much about them, but about someone else.
It's a very balanced OT3 overall too, with each one balancing out the other two.