Hyborem can be saved. If he stops worshipping the veil he can build the gate. If Basium comes he won't declare war on him, Basium hates the Ashen Veil, not the infernals. Or more specifically Basium hates Agares, who the Veil worships. If Hyborem is redeemed Basium will leave him alone.
The AI will never do it (summon Basium with Hyborem) but I left it as an obscure option for human players. I figured someone would want to run a Basium/Hyborem team.
I can justify most combinations, the point of FfH is to allow them, and the civs and religions were picked specifically for their ability to be interchanged with each other. But I must admit, the Cult of the Dragon Dragon Slayer is pretty funny.