1. You have to interact with Hinata when she lies on the couch and try to massage her many times, until she agrees. then you can buy a manual from the shop. then you read it in boruto's bedroom. red ! mark.
2. You don't. It's a love only quest atm, she won't wear it for anything else (yet).
3. You will never /win/ the best you can do is get her to admit defeat. Pinning her against the wall when she interrupts you doing pushups, after you are strong enough. You can beat her in a squats competition.