Just to reiterate what has been said above that neither Expedia nor BA would have had any insight that the flight was overbooked, only Qatar would have known that.

In my view Qatar have really let you down here, they should have offered rebooking at Bangkok as the operator of the overbooked flight.

The entanglement of BA and Expedia didn't make it any easier as Qatar used that to distract your efforts to find a resolution, and Qatar call centre staff probably had their hands tied as it wasn't their ticket (and culturally they are not encouraged to read beyond the script).

I suggest Qatar ground services in Bangkok have managed to let the ball drop big time here, they could have endorsed the ticket themselves and got your people where they needed to be.