This is all about boosting the Thai economy not about making sense. Soft power means legit schools get money. The extra time in Thailand got so many people to sign up for these. The business being legit has a lot to do with not getting any income from Thai sources and not looking like you just borrowed money to show a balance.
You won't need to show proof you have purchased a ticket, and they wouldn't expect you too. Just look for an ideal flight and date assuming all works out and put that one down in the application.
Yes I saw an interview with foreign affairs and they said 1900 plus showing proper documentation. But if you leave and come back you get six months each time, no docs.