Yeah, me too!
I do think this is one of the tricky points. At one point I was actually wondering if this was a structural flaw in the story, because Elias becoming a monk seems like a big change, but if you look at the Problem elements they fit much better with Petros as a change character.
So to borrow from the other thread, here is an argument for Elias actually growing into his resolve:
People think I am immune to guilt, he thinks ... Was he just that good at ignoring his conscience? And what had changed?
Then, we get Petros' reaction to the news of Elias' becoming a monk:
Another overdramatic gesture, Petros thinks, but maybe for the best.
In other words, at least from Petros' perspective, even in becoming a monk his cousin is just the same old Elias.
Contrast with a couple of lines from Petros:
The realization sinks in: he has come all this way, has killed a man, has come within inches of death only to discover things that should have been left undiscovered.
and then later as they're swimming:
"We have to abandon it," says Petros. He thinks of the diary, the surveyor's maps; the last photo of Joanna's father .... "Let it go," he says, this time more firmly.
... which I am now arguing are examples of his change.
So I'm going to stick with my argument that Elias is actually growing into his resolve and Petros is changing, but we should look at the Problem Elements for sure.