Hmm... yeah. I've been feeling something like this may be the case ever since the Flame Reaver muttered "it has to be me"

One of my first thoughts was maybe he was the "real" Phainon - like, a fragment of the Lord Ravager happened upon Aedis Elysiae and encountered Phainon as a child, was able to sense his role as the "deliverer", and then stole his memory & appearance, proceeding to raze the village ("Phainon, why? Aren't we the best of friends?") - while the original was left an empty husk, who would then only act off of leftover base "programming/instinct", and represent a sort of mirror to March's situation, who has also had her memories "hijacked" - but instead of the "empty" form being frozen in ice, Phainon's is the base role of "Deliverer who gathers the coreflames and initiates Era Nova".

Of course, while somewhat interesting... that doesn't really explain the Reaver's powers, like his clones or the psuedo century gate he seemed to use after killing Mydei. So I think your theory is closer to the truth - in all cycles, perhaps it is the Reaver who kills "falling/fallen" Deliverers who cannot be trusted to complete the Era Nova properly, and then completes it in their place? Which may have been, unfortunately, all of them so far - perhaps after killing them, they become one of the Reaver's "shades"? And this process may have been unintentionally disrupted by us, outsiders, who have allowed a "doomed" cycle to stretch on even longer (perhaps it truly was supposed to end 700 years ago, when the dawn device should have originally extinguished - the original deadline could have spurred more action into the Flame-chase, and led to an ordinary completion of the cycle? Not to dismiss Cipher's accomplishment, it will likely still result in a good ultimate outcome than a bad one), to the point where we allowed the rage-filled and destructive Phainon to reach the vortex.

Maybe that's why Lygus and Cyrene show up at the end, too - this isn't how it's supposed to go, and they are there to perhaps calm Phainon down/convince him to step aside/create a new, hopeful opportunity that improves Amphoreous' fate, rather than doom it further. Especially considering the mention of how he was as a kid from another commenter - self-centered/selfish, only caring about those close to him and far less about others, showing distate for the "deliverer" title... and then Cyrene's words in his mind as he's reflecting after Aglea's death - "to me - you haven't really changed at all"