I don’t think a charge back is what the OP is looking for here. Apparently the tickets are already in the process of being refunded. A charge back will only mess up this and should only be used later if tickets are not refunded.

To the OP: As the tickets are being refunded, there is no way Exp.,BA or QR is going to reinstate the tickets and rebook the flights. Period.

To have the issue with 2 pax at BKK resolved now, buy new tickets (not with Exp or BA but directly with the airline operating the flight). Then you can try to get some kind of compensation afterwards (but that is def. not going to be easy because of the finger pointing and uncertainty about the actual reason for the tickets being cancelled and pax denied boarding).