But that very theory both establishes Kaworu's lack of trustworthiness and suggests that Shinji is not responsible for 3I, which means he has plenty of options for a happy ending in Final.
Considering it's a bit ambiguous as to whether the stinger/next time preview at the end of 2.22 is still canon. . . I'm a little less inclined to believe that.
How is it ambiguous? Apart from pie-in-the-sky speculation by yours truly I've seen no indication anywhere that it is not 100% canon. Like, none. Zero. Zip. Nada.
So? He's the domino that started the whole thing. No Shinji causing instrumentality to save Rei, no Impact and no coagulated landscape. If your actions caused a devastating snowball effect that resulting in a lot of innocents dying, how do you think people would see you?
Keyword "if". You yourself just provided a credible scenario wherein Shinji is not the domino that started things, and that places the blame for all of it squarely on Kaworu. That rather handily shoots your whole doom and gloom scenario in the foot.
True we do see fresh water in 'Until You Come To Me". . . I'm sure there are small pockets of civilization arable throughout the world. Just like there was a small pocket of water for sea life in one aquarium. But will a small pocket of comfort really undo all the damage he's done? especially after 3.33 hammered home how there was literally no way to undo it?
We have no reason to think it's a "small pocket" vs. the status quo for everywhere outside of Hakone, Japan. You don't build and maintain the U.S. Pacific Fleet (several times over, actually) with a "small pocket" of civilization. You need a fuckin' superpower to do it, and that means millions of people, industry, arable land and fresh water, and so on and so forth.