Well, since all the agents were unable to confirm the seats I was seeing as available online, I ended up waitlisting those segments, and interestingly enough, SQ sent out an e-mail about 6h later that all waitlisted segments (incl. 2x F US-HKG) had cleared.

So not sure whether it was a systems issue or just a bunch of incompetent agents, but either way, can't say I'm particularly impressed, although I'm happy everything is no confirmed.