The reason why the Chinese extras were used at Royal Ascot is because Royal Ascot was doubling for Shanghai Pudong Airport. It is very, very difficult to shut an airport down for filming - especially a busy one like Pudong. It's a common practice in the film industry to do this.
As for the sets in Pinewood, this is again a common practice. The safehouse and the art gallery in QUANTUM OF SOLACE were both sets built at Pinewood, as was the interior of the Andean Grand Hotel. Not everything is filmed on location because it's simply too expensive to fly out to locations and there are no guarantees that they will be able to find a place in Shanghai that would fit the needs of the script. So they build sets at Pinewood instead.
Yeah I know everything isn't filmed on location (your Captain Obvious picture would come in handy here
) but my "source" specifically said they were building sets because filming in Shanghai had been scrapped. But this is not a watertight story of course, he could have misunderstood, Chinese whispers (no pun intended), massive pinch of salt etc...
From what I understand, they were going to film there for about one week. So that does not gel with this person saying they were expecting to film there for 'several weeks'.
Other parts of China, according to other information (and to some extent the official statements), would have had the crew there for 3 to 4 weeks. At this stage, I don’t see any real reason to think that this plan has changed.
Interesting, although if they had scrapped that one week of filming, that would be quite significant wouldn't it? Still, fingers crossed that's not the case