Great theory but yeah nah bud ain't Mephisto. They just (spoiler) met on two different occasions in the far past due to various reasons

I think I remember this is exactly the bit he finds out

If I remember correctly, Tyer will basically make Agavert outperform Trevaga, forcing her Dad to disband his guild due to them not able to get any more commissions and join Agavert (though very very veeeeeeery reluctantly with much resistance on his part). After some back and forth, Tyer and her Dad will get thrown back to Korea some time after both the Dad and Tyler left, where her Dad finds out about him having a daughter etc though he doesn't know what to make out of it at first. His memories will only return at the very end of that segment.

No one's gonna die except kinda KDJ but not actually because of the agreement between him and Secretive Plotter like the previous comment said.

KDJ will get booted to another timeline instead.

Also the incoming battle against Surya is actually kinda fire in the novel so def looking forward to that