Ngl, that's not a bad idea
That just makes Rein vs Lay even more of a possibility
- Belief isn't necessary, the tale just has to be spread
- Avos still formed despite most demons knowing who the real demon king. The rumors about the demon king all still point towards Avos Dilhevia.
Aside humans spirits and demons, the tale of the demon king was spread in the divine realm by Nosgalia and there isn't a God out there who doesn't know who the demon king is as well as his name. Kanon changing history didn't affected the Gods yet Avos was still formed, who is the real one or not doesn't matter whatsoever, all that matters is the tale existing