I know there are articles floating around that say they'll offer only 170 seats, and nothing more, but I'm skeptical that it'll be that few.

However, even less likely than SQ blocking that many seats is SQ refitting the A350 specifically for SIN-SFO.

As the SQ press release says, the aircraft on the SIN-SFO route is configured with 253 seats, and some seats will be blocked due to payload restrictions that depend on headwinds. That is to say, under some favourable weather conditions, they may not even have to block seats at all. (Though admittedly this will probably happen less often than not.)

This sounds to me as most consistent with my initial claim of SQ not refitting their a/c nor blocking 86 seats on a regular basis.