Umm, この世界の最大の不自然に is an adverbial phrase modifying 決着をつける (to settle [a dispute])...
Ah, I don't know. This is a pain. It's not like I particularly translated this for your sake. Don't get any weird ideas.
Who are you!?You...just like that.Why are you calling me John...When the joker was flipped face up......somehow or other....it could have been too soon or too late......the position could have already been cut off......or beforehand...at last we are able to meet!I don't know why you switched off with Kyon, butShe ended the biggest mystery of this world,