Because Australia and Thailand have a double tax agreement.
So, stuff that is taxed is not double taxed. On the other hand taxes in Thailand are lower - so you should figure how to get taxed here instead of being taxed in Australia. After all you are a tax resident here anyway if you stay 180days or more per year.
Hard to say. I stayed 2 years on visa exempts. The exempt is for tourists. A MTR gives you 9 month, close to 270 days. Roughly the same as three visa exempts.
Technically it can. Since COVID. And that does not every HR department know. Then: usually you had to have something like 21 days (could be 14 days, depending on visa type) left on your entry stamp. So only a few days (5 working days) open to hand in the paper work.
Now with 60 days entry stamp that would be no problem, of course.
Yes, he is a bit naiive. Probably he was bored when he asked here, and did not think about it. Because his problem is not how to get to Thailand, but having no passport.