every airline I've flown with from the UK to Thailand has asked to see a return ticket pal. If you've got your Thai ID card just show them that, if you haven't they could refuse boarding because you wouldn't have a return ticket.
Are they registered in Thailand and have their ID card? If yes, then yes, if no, then you need to go about getting them registered and their ID card first.
if you have a bachelor in education or higher and 1 year teaching experience in your home country or 2 years in Thailand you only need to sit the Krusapha test, you do not need the 9 month top up course.
although I am not from Aus myself I know 2 people that have sponsored their partner, one was told by the agent doing it for him he needed a minimum of 30k savings with regular income going into his bank, the other was told 20k with regular income, both had no issues getting the visa.
You will need to do a degree/masters/PhD at a university and find a university that will support the applications. Language/Muay thai/Cooking etc schools can not support dependency visas.
you can get 2 license waivers which is 4 years, it is possible to get a 3rd waiver if you can prove you have enrolled in a top up course if you're not a licensed teacher in your home country near the end of your 2nd waiver.