Of course these would be real products leaving for China. We already have a ton of excess capacity, so I don't see a problem
in this hypothetical scenario. If the economy has significant idle resources it really isn't important if demand outpaces productive capacity. Our economy will expand to accommodate Chinese demand and we'll absorb these idle resources and turn them into productive ones.
Where is this excess capacty that can be put to use merely by increasing the aggregate demand? Come on... this kind of free lunch simply does not exist in the real world. And if it does, the FED+govt could and should be utilizing it anyway and the chinese or whatever would take that off the table. You are ignoring the opportunity cost and the fact that the excess capacity whatever you mean by it can't be that simply utilized if it could, it would be utilized already.
Also, I don't believe that increased AD will lead to anything in the long term. It's a way to balance fiscal cycles, not a way to get you into permanent prosperity. As far as I know the long term GDP numbers performed better when there was "lack of AD" aka. deflation in the long run. It's silly to think you can boost your economy in the long run by boosting AD by ever increasing amounts. I don't think there is much evidence that this works generally. Likewise there is little evidence that there wouldn't be cycles of unused excess capacity if the China dumped it's dollar nominated assets, in fact, such scenaro would probably cause a other recession as people would have to move from service sector into manufacturing where the added demand was. The businesses could no longer afford the imports etc, it would be a classic supply shock with demand stimulus, resulting in high inflation and unemployment.
In other words, the short term economy would most likely not pick up in response to China dumping it's dollar assets, in fact the opposite. And the long term economy would not be boosted either. Why not just admit that when someone dumps your assets it's probably not going to be a great deal. The worry is entirely warranted n my opinion, but will it play out, and how quickly... that's the real question.