It was more the how than the why. To me it didn't feel like there was enough tension built between Hawke and Seyfried, although the magical mystery tour was a big step, that last pre-moving/pre-ceremony meeting and conversation between Hawke and Seyfried didn't really convince me enough that they'd had this "bond"/"feeling" that was just waiting for someone to make the first move or a recognition of reciprocity to take things to the next level. It there was a bit more tension in that 1 scene, I'd have more leeway with the ending as it was.
2nd part of the how, I thought the "suicidal angst" angle, and being "saved" by love was a great concept, it was just how it happened, again like above, that going full blown passionate making out all the way to the credits didn't do it for me, in part because of his reaction to that, "we'll say nun", about their past sexual encounter/non-platonic relationship or if he used different words with her imo. Maybe I wanted Hawke to be crying tears of joy throughout their make out scene, taking breaks because he's overwhelmed with love, heavy panting and staring into her eyes maybe, "heavier" blood seeping through his cloth, a look up to the ceiling to show his "faith" fully restored, stuff like that to really sell the "experience"/"epiphany" that's happening.
I disagree that the other 2 options would have been bad per say, it's all dependent on the how.
The act of sacrifice, would have been as powerful if he'd played a twist by taking his suicide vest to say...Balq's family's house and blows up his wife and kids with some monologue about "suffering not being felt till it hits you directly" or "he will finally feel some of the pain his sins have caused for mother earth/it's inhabitants" or something similar to that, and obviously correlative imagery of his wife/kids dead and him arriving at the scene and fuming with (self? if he knew it was "pro-environmentalist Hawke" for sure)anger and defeat.
Self-flagellation would have been the toughest to sell. Even if it was an act of suicide or the movie ending with him in a mental hospital, it just wouldn't have felt right or sent a message in either direction that it needed to for the movie to end with real closure. You're right on that.