It's not the Thais, it's the airline at your airport.My wife even checked with the airline in advance. Like I said we've already done this twice. They can see your return ticket. Last year Air Canada and the person at the check in knew we were getting an extension.
This is our third year spending 90 days leaving from Vancouver. We've never had a problem, as long as you have your return,they know you can get an extension.
We will be using a 60 day and a 30 day extension.We have been 7 out of last 10 years and have never overstayed. Last two years we have stayed 90 days this year we want to stay 120. Thank you