The answers to most of your questions can be found on the Krisflyer website, and the award charts that are downloadable from the site. There are good tables that show redemption rates (and you can see availability on SQ services, not UA services). One ways awards can be booked (at 50% of return).

As far as zones go, China and Korea are in same zone, so a routing SFO-ICN-PEK would be without doubt OK. (SQ flight SFO-ICN). Not sure about via HKG.