it’s well documented that some people have been refused boarding on a flight to Thailand, because they did not have a return ticket. It’s entirely up to the airline and the personnel checking in passengers. It’s a hit or miss game.  But there is an easy fix, step out of line, purchase, an onward ticket, get get back in line and board the plane, hold that ticket till you pass through Thai immigration, then cancel the ticket. 
I want to thank Benjamin Hays, I followed his directions for the in-country non-O Visa. I did it myself and it wasn’t that difficult since most of the documents needed were within my reach.