I completely agree. The moment when Amir and Assef confront each other as adult is, as you said, when Amir's "past and present conflicts collide". It really brings to light how Amir got a taste of being defended by Sohrab in the moment when he was completely ready to defend Amir, just like his father, regardless of the consequences. It gave Amir an opportunity to defend Sohrab against Assef just as he should've done, or should've been willing to do, for Hassan.