I like your theory on Joel being quickly accepting Hannah's suggestions over his own plans. And also her cutting his talk back at the campThe Smile
On P.6, "You always hide something with that smile". The mid-page panel has Joel with a forced (almost, pained) smile, where he's quick to agree with Hannah and disagree with kid Kaikashi after he specifically says "this is also your first time", suggesting he knows something (if you take it at face value, maybe he's just supporting his childhood friend, but... maybe he knows it's not her first time and it's not his first time).
As for the word "will", maybe it's a callback to Joel's own word when beating down the trashtalking man? He said "I will carry on your parents' will", which the parents themselves are apparently someone who protect ppl. Hero themselves?