1. Yeah they could try, but it'd be a real bad idea. While the USM has the PLA beat on tech and training, they'd never be able to replace their losses fast enough. And more to the point, they'd have absolutely nothing to gain from it.
2. It'd help, but I doubt it'd decide it. Occupying China wouldn't be much fun either, so supply lines would have a lot of trouble. I'd imagine the USM would take pretty massive casualties, but the PLA would take a hell of a lot more.
3. Pretty darn effective, considering Aircraft servicability, pilot training and real-world experience are pretty thin on the ground for China. Also take note that the PLA-AF is still largely made up of totally obsolete aircraft like the J-7 and J-8. Their Flankers would pose problems but there aren't enough of them and they dont have the same pilot skills as the USAF, so while the USAF would take a lot of losses, they'd rack up a pretty itense kill ratio against the PLA-AF.
4. That'd depend on the reason for war. By far the most likely reason for actual armed conflict between the USA and PRC is over Taiwan, whether or not that would be a protracted war of attrittion is impossible to know for sure but I doubt it. I'm sure a lot of people in both the USA and PRC wouldn't think Taiwan worth fighting over, although there'd be a lot of recruits from both populations none-the-less. Seems to me there are a lot of idiot Nationalists in both countries that want this war to happen.
5. Depends on the resources given to them, even if they would succeed in actually invading the Mainland (Not gauranteed) it'd take a pretty super-human war effort to sustain supply lines like that, and controlling an occupied population of hundreds of millions may well be impossible for any one nation to achieve. I'd imagine both nations would be too busy dealing with the after-effects of a nuclear exchange before the USA would try to invade and occupy mainland China.
6. This will never happen, the USA will never be invading the Chinese mainland, because it'd be doomed to failure, either in the short term or the long term. Both countries would be economic, if not physical wastelands from sustaining what would need to be the greatest war effort in Human history.
The most likely scenario for war between China and America right now is over the Taiwan straits. Say China has some domestic troubles, or the Taiwanese do something very stupid like declare independence, so China uses their (somewhat limited, but growing) power projection abilities to invade the Republic by Sea and Air. The Taiwanese Military is apparently having some troubles with incompetence, inefficientcy and corruption, which weighed with the massive superiority in numbers the PLA has, and it's growing technical superiority, means they would have a real hard time stopping the invasion, if it's even possible for them to do so. Thus the USA would have to decide between humiliation or fighting a very expensive and possibly futile campaign to drive the PLA out of Taiwan before they secure the Island. The US Navy would most likely devastate the PLA-N before too long but by then it might be too late, as it'd be pretty hard for a nation even as powerful as the USA to take Taiwan back if the Chinese had it secured. So after that it'd probably degenerate into the USM shooting the PLA-AF out of the sky and making selective strikes on the Chinese mainland, while they tried to isolate the PLA forces on Taiwan and force a negotiated peace.
Only problem is, the USM would take a lot of casualties and it's cost both nations so much money it's hard to say for sure they'd go through with it.
Just a theory anyway.