AFAIK so far DL hasn't done the analogue of starnet blocking, but DL miles can be used on partner award tickets only if there is "low" availability. KE has many blackout dates, which DL interprets as being systemwide, so that effectively DL miles cannot be used on KE for close to half of the year.

As you probably know, China Southern has good service within China (which I have experienced) but their TPAC (CAN-LAX and VV) is widely rumored to use extremely tight seating in coach.

Have you considered StarAlliance? In addition to the TPAC flights on UA, SQ has a nonstop HKG-SFO as well as lots of connections through SIN, although UA SWUs can't be used to upgrade on SQ and SQ status would not lead to UDU on UA/CO flights within North America. SQ is a great airline, but don't expect OPUPs, good handling in IROPs,
or much post-arrival service.

Do you primarily travel to CAN/PEK/SHA-PVG or must you reach somewhat smaller airports (maybe cities with populations of only four million?) within China?

I'm surprised by your insistence on avoiding CX from HKG and I wonder if perhaps you're overreacting to one very bad experience (or the recent runway strandings upon arrival at JFK and the diversions to Canada). The grass might not be greener elsewhere.

To at least some extent, your decision should be based on your annual spend and not just miles flown. SQ is particularly noted for reserving benefits for high spenders on their metal, while UA offers GS with special benefits to those that initially hit their spending requirements and then fly a certain number of miles in high fare classes.