From Australia, Yep. Be a crappy way to start your holiday if they made an issue of it. But then I have flown from many other places into Thailand and never been asked. Just book a cheap onward or 'rent' as someone suggested in the comments.
Be a crappy way to start a holiday though. Most times people get away with it at entry into Thailand once they get past the airline check in. Most likely to have problem with airline and not in Thailand.
Doesn't matter if the ticket out is with another airline or destination as long as it is a ticket out. The airline can check you have a ticket out before boarding and so can immigration at the airport on arrival. A return ticket is not necessarily required, onwards ticket is fine.
Yes but the return ticket is supposed to be for 30 days and not 60 days. The airlines don't take into account the ability to extend for another 30 days. They want to see a flight out before the end of 30 days or 60 with a tourist visa.