It's probly not so much thai immigration that'll be bothered more than likely the airline your flying with, got asked for onward travel when I cam to Thailand 15 days ago ad had a single, pressure they made note I just said I'm booking today before I borded and was sufficient but booked a fake ticked incase thai immigration asked and I had the metv
If I'm not wrong that would be either a b1 or b2 visa no? Once the application is done you can apply for an emergency appointment but wait time in bkk is 15 days I belive