WarKirby
Arty person
Using that rationale Cassiel should lose population from his cities as people flee to the Illians in such a scenario rather than suddenly compromise one of the civ's basic flavours (or lack of flavour). As I mentioned before why shouldn't he suddenly choose another religion such as one of the ones that would grant his civ good alignment (which is protection enough for now until the AC is changed)
Because going Order might keep hell out of his territory, but it's not going to kill the hordes of demons that simply walk over his borders. Those Demon-slaying crusaders might seem nice, but Combat IV manticores will eat them for breakfast. Hyborem doesn't care what religion you are when he comes for you. Only if you have a big enough army to oppose him, which is often not the case. In a recent game I played, AV was founded early, and hyborem appeared in a vast uncolonised area to the south of the main continent. He had a continent spanning empire and crushed everything around him. Only my Vast tech lead and (probably overpowered) doviello army allowed me to stand against the 40-champion stacks of doom hyborem was sending at me. Every AI between me and him was annhilated quickly.
Adopting white hand is more than just taking on a religion. It's also taking on the protection of the illians. It's relying on Auric Ascended to stand against the forces of hell. No other religion offers a literal god to fight on your side. It's not a "good" option at all. It's choosing the lesser of two evils. It's the last hope when the bannor are crushed, the holy city of Order lies in ruins, when sphener and valin phanuel have been annhilated.
For the case of this to happen in game, the illians are going to have to be powerful enough to vassalise others. As far as I'm aware, AI civs tend to surrender to whoever they feel is most likely to be able to protect them, strongest civ generally. So it would work perfectly in a case where the illians and infernals are the two largest civs, and the infernals are on a warpath (as they frequently do)