One thing to note and this may vary from country to country but as the TM6 which showed your departure date is no longer used for entry into Thailand, the onus now appears to be at your departure point. I know that at the Auckland (NZ) airport I would not be able to board a flight to BKK without either a return or ongoing flight within the 30 day visa exempt period. There are ways around this with a 24 hour temporary flight ticket or a flexible date flight.