I may be a bit bias as I'm in HK

I think your best bet is via HKG as there are plenty of BKK flights out of HKG. Quite a few of the middle eastern/ europe airlines transit there. Not sure how's the SEA-HKG would be thou. Do beware of long weekends as HK people tend travel during those days and there are always some holiday in every month

I haven't done out of BKK before but I would assume that the ground staff there maybe less experienced with dealing with ID tickets as in some "less international" ports that I've been. They always give me that funny look like "Are you allowed to fly with us with this ticket?!?!" before asking their supervisor. Non-rev out of HKG is cake, if the load is light, they will check you in straight, if not you go to the standby counter depending on which airline you are flying with