@Steve ******
you are of course correct regarding the TDAC Arrival card. That's why I left the departure date blank.
However the logic in the context of the airline refusing to transport the passenger due to lack of an onward flight would be that Thai immigration does not require this per se so why would the airline refuse when there is clearly no repercussion to them if the passenger has no onward ticket.
And of course you are also correct that the IO can ask whatever they like and let you in or not based on your answers.
Going back to my original comment would you be so kind to provide the source that states the requirement for a DTV holder to have an onward travel. This would be helpful for everyone in the group where you have found that requirement. Thank you