im just not sure who’s in charge here of actually deciding whether it’s allowed to waitlist me. Etihad or Bangkok airways.
Eithiad would be the one to change it in advance. PG would be the airline to hande day of departure.
I would bet that the real answer will be that Eithiad would be a pain to deal with. But at the PG ticket window if they have a seat, and you have 1000B (since you are in F, more expensive than the 500B for Y), then you will be on your way to Koy Samui regardless how the rules are written.
It's not like your are in the States after all :-)