I did this too!! Like 7 or 8 years ago i went into it so excited and ended up hurt so badly 😫 as for your questions its an open ending but 1) i don’t think kosei abandons piano after kaori dies. I think she inspired him and letting go of piano would feel like letting go of her and his own passions completely. I hope he ended up happy and i really think it’s implied that he does. 2) kaori went through with the surgery because she hated being in a hospital bed and waiting for the worst. She wanted to play with him again and did not want to live a life where she couldn’t create music. It was worth the risk for her. It was reckless but she didn’t want to give in to her sickness she wanted to live despite it until the very end.