Looks like my mileage woes were for naught – my Thai Airways miles were accrued on a 1-to-1 rate for KrisFlyer, so I’ve actually already achieved KrisFlyer Elite Silver status! The silly travel agent apparently quoted the miles for one-way flight, so I don’t even need to bank on the return leg of the journey to get my miles.
In this case I guess I’m quite happy with the outcome, but if I’d decided to buy the (more expensive) SIA ticket just to get Elite Silver, I would be really annoyed… Oh well. Good thing it’s turned out the way it has, then. =)