if you want a stopover, your only option out of SFO is through HKG. I would suggest being a bit flexible and try looking at LAX too, I know the drive is long but it would open up LAX-NRT/ICN-SIN for you to look at too.
SQ releases award space 355 days out at a specific time (can't remember what time sorry)
of course its always better to book ASAP.
SQ does not release J/F award space to UA afaik so you would have to watch SQ.
regarding upgrading, the fare buckets you'll be able to upgrade from are usually really expensive on SQ fyi