It's happened to me a couple of times. I've always sat it out and always got the Approval, even after the due date. If you're worried, and it's convenient, go to Immigration, but the worst that can happen is you're fined 2,000 baht next time you have to go there. I bet it comes!
Not impossible. You need to show you have enough to cover her trip, so if she's just coming for Christmas and staying with you, it doesn't need to be a lot. The 'reason to return' is more difficult, if she doesn't have a job. UKVI may suspect that she will remain in the UK with you, unless you can satisfy them that she will return home. If she's travelled to other countries and returned it will help.
If she just wants a Visitor Visa and you are sponsoring (paying for) her trip, it's you that has to show you can afford it. Her bank balance is irrelevant. However, she will need to demonstrate a genuine reason to return to Thailand after the trip, like a job.
A decent TEFL qualification like a CELTA is important for proper language schools where you'll be teaching all ages and some other nationalities but you'll need the degree for a work permit. Of course, some schools find a way around this 🙄