So it seemed this remake they are keeping the main point of the storyline. I am glad they keep them because those events are essential to the story.
Giving back the harmonica moment was needed for both of them to move forward in their relationship. For Sila, he knows that Mintha was ready to move on but before that I don't think he even realized that part. He in a sense took her love for P'Tor for granted. All this time, she has been after him just for that, to admit that he is indeed the person she has loved but his refusal not only hurt her emotionally but has cause her physical pain as well. The look on his face says it all...he was going to lose the very person he didn't think he ever would since he resurfaced.
For Mintha, it was about letting go of a past that she remembered to be so beautiful and accepting the present that's just straight up ugly. The person she had stored in her heart no longer exist. He became a person she doesn't know and by giving him back the one item that reminded her of P'Tor she finally gave him what he wanted to be Sila, and for her to leave both Sila and Tor behind.
Sila will be the one that has to work harder to get her back. To me, once Mintha decided she no longer wants any part of him is actually where the story begins for the both of them.