First of all, I just unfollowed you on twitter.
SECONDLY, I don't think that Amir would have sought out to atone for his wrong doings without the phone call from Rahim Khan. Throughout the novel, Amir was given ample opportunity to make the right choice in difficult situations. However, he never chose correctly because he feared the consequences of doing the right thing. For example, when he saw Hassan in the alley about to be raped, he had the opportunity to tell Assef and his two friends to stop and take the kite. Instead, Amir didn't do anything because he was afraid of the consequences for defending Hassan. He ran and hid away in fear of what Assef may do to him if he saw him. Amir also had another "opportunity" to make the right decision by not planting his watch and money under Hassan's mattress. This wasn't so much an opportunity to make the right decision, but a choice within himself.