>>55061501They literally saved Arven, twice even. Once in the "main" timeline where they not only helped healing his beloved Mabosstiff (the only family he has left), they also finally find out the truth he didn't want to acknowledge about his parent's death. He could then accept the ugly truth and move on with his life instead of dwelling on his trauma and becoming bitter or full of resentment.
And then a second time in the separate timeline they triggered by exchanging the books by the crystal lake. The fact that the professor gave us their unique copy of the book means that they will not use it as a key for the time machine like they did in the "main" universe. In fact, I believe that talking to the professor about their family made them rethink their obsession with the book, which is why they decided to part ways with it instead. While we know the professor did receive Briar's book from a child at the lake, it is not known if it was the MC. Fact is that they did not give the book away, which then lead to the events we witnessed in the game. The professor who did give the book away might've made very different decisions once they were back, possibly spending more time with Arven.
As for Kieran - it's complicated. If anything, Kieran learned important lessons through the MC and went through some significant changes. If that didn't happen, he might've forever stayed a meek, shy kid who would forever stand in the shadow of his sister and never grow a spine.