When I tried to get (and briefly, incorrectly, had) an SQ award ticket with the return leg originating in PEK, I was informed by a knowledgable and helpful agent that the agreement between SQ and DL excludes mainland China from the list of available destinations for award travel available to DL SkyMiles members. If that is correct, and I assume it is, that means no award travel to PEK, PVG, CAN, or SQ's other destinations in China.

Now, even if you could do it, taking the KCR makes much more sense unless you really want to maximize time in the air/onboard SQ. If you stayed on SQ 1 SFO-HKG-SIN, by the time the Megatop would be airborn to Singapore you should be well on your way to the KCR station. By the time it touches down at Changi, you should be in Guangzhou. So you're saving the backtracking leg SIN-CAN and all the time involved. I suppose you might have tried to get on a ULH non-stop to SIN and then pop up to CAN, but it's pretty much backtracking no matter what you do.

Take the KCR. Spend the time you save skulking around Hong Kong.