Many times it is the airline checking you in that demands it. I’ve had it several times and no amount of explaining to them will work. They won’t let you board without it. Had it in both Barcelona and Doha. Just bought a cancellable ticket and showed them.
As far as Thai immigration, I’ve never been asked and that wouldn’t make sense
Been in and out of there 6x in the last 2 months and never any issue. In fact the immigration officers already have a 6 month stamp with date set to process the dtvs
yep, you show the visa and they say “only resident visas don’t require an onward ticket”. Been through it multiple times. Barcelona via Qatar airlines was the most recent