Now I feel dumb.
Probably closer to Kerisgame flavor, from the Solar's PoV, but with less stability. But with an ending closer to the one in Green Sun.
Like, for example, Infernal loses control, Solar comes out, Solar proceeds to be confused, Solar does whatever, stuff happens, and the Infernal wakes up to find his whole coven dead, the city on fire, and himself napping on the king's throne.
The thing is, that's fundamentally incompatible.
A Past Life possessed starting Infernal is still a starting Infernal. They will not be able to kill their entire coven unless they're already the party combat specialist or otherwise considerably more killy than their allies. Any mechanic where a starting character can suddenly gain hundreds of XP is a NOK mechanic for the standard case, doubly so when this boosted character is also ideologically inclined to fuck over the entire party. I'd ban such a mechanic from my games because any player taking it is basically a time-bomb that can screw over the entire party and thus it's antisocial play. And that would make me sad, because I like playing around with Past Life stuff - but making it a way to go Evil Super Saiyan and ruin everything is not getting in.
No, the "delusional Infernal" is the only way to meaningfully do it and actually use it in a game.
Please note, too, that the AGSITV occurrence a) nearly got the Slayer killed, b) was mostly in the face of mortal opponents, and c) was against enemies who knew nothing of what the scary green-glowing murderer was. It was not against Dragonblood-tier opposition, let alone against Celestials.