You need to ask the flight company. Some of them don’t let you board the plane of you don’t have a return flight. A dtv visa doesn’t have anything to do with a one way ticket
Pretty easy. Just click an amount you want to use to refill your account. The higher the amount the longer it will stay active. 20 baht adds 30 days. And you don’t lose the money. Because in the app you can buy packages with data.