Apply for the visa at a Thai embassy or consulate in Australia. They will give you all the information you need to make the application as seamless as possible. My advice would be to shop around and get medical insurance in Thailand. Extremely risky not to have it. If something sudden happens and you can’t get air lifted back to Australia, it can be ridiculously expensive. Something as simple as a mosquito bite or a car accident can have dire consequences. Good luck.
I previously have booked tickets with Thai Airways which have a departure and return date. I have then cancelled my return flight, and booked another date, sometimes 2 to 3 times. There was a fee of 500 baht each time I did this. Not sure if it is still the same as I haven’t done it for a while. Maybe contact Thai Airways for confirmation. I’m sure it would be cheaper than a flex ticket. Good luck.
You could book a return ticket with Thai Airlines, putting an approximate return date, then change the flight when you know your return date. Not too expensive.
My understanding is that he can get a passport. He has to do it in Thailand, and be included on your wife’s family book. Unfortunately, being the holder of a Thai passport, he would be eligible for military conscription.
I think it depends on age. My 17 year old son has been told he has to be registered in Thailand before he can get a Thai passport. He was born in Australia, they gave him a Thai passport in Australia when he was young.