I don't think the airport idea is good.

The airport can have unlimited powers to put you on any of RJ flights within their control (ie: departing from that airport and within the next 30 hours or so), but they can not rebook you on another carrier. In all cases, rebooking on another carrier will require going through sales (which is a different department from the "airport operations" and then also head office as well (the sales office can only propose the route to the HQ and wait for their answer).

Again, I would suggest emailing the BKK office. They are aware of the cancellation know the typical permissible routings and are pretty efficient. If you send them a list of 2-3 different itineraries, preferably via different routes, it should speed up things a lot. Also mention straight away that the OTA sent you to them so that you don't waste a day on that topic.

For ARN, I would say:

BKK-HEL-ARN - about 10% chance of approval;
BKK-DOH-ARN - about 60% chance of approval;
BKK-DOH-AMM-ARN about 90% chance of approval