Democracies are slow to respond; the system is supposed to prevent corruption but it also means that it must run its course through many people and checks. The US didn't react quickly enough on Crimea or Georgia. The US still can't form a cohesive reaction on the SCS. That you think they can make rapid decisions in this context is unsupported, especially when it comes to a decision that they know can provoke nuclear war. And to be honest, China would much like to keep the US out of the conflict, but if the US insists on being in Taiwan, then for China, there's nowhere to go except to strike the island with them on it and prepare for full scale war. Keeping them off is to protect them from throwing away their lives there and making things difficult for everyone (USA, Mainland China, and ROC); it is NOT because China can only act without them there.
You mean that a missile might miss a military target and destroy a city? That might happen, but not often though. Chinese missile accuracy is very high as it was greatly valued since Mao Zedong's time as an asymmetric warfare route to develop; China is the only country to develop a land-based ballistic missile with the accuracy to hit moving ships thousands of miles away. It would be fantasy to think anything other than that the vast majority will successfully hit their targets. But if China decides to blanket bomb 2,000 missiles over Taiwan, it won't be much of a difference anyway. If it fires a few hundred, it may be to cripple Taiwan's military, but if it fires 2,000 then a red line was probably reached where it was decided that it would be too difficult to take Taiwan with any preservation of the island's resources, so it must be completely destroyed to show the world the fate of traitors. Then it would reduce the issue to who owns a smoking rock. It's not ideal but most Chinese can live with that. They cannot live with Taiwan's independence.
That you think the Chinese are not prepared to go to nuclear war for Taiwan is precisely why you are too ignorant on the Taiwan topic to make any meaningful contribution. You say it's a complicated topic but you clearly don't understand what that means. China's leadership has declared nuclear war before Taiwan's independence and the Chinese people will not tolerate the government backing away. I've basically never met a Chinese person who grew up in China who thought that Taiwan should be independent under any circumstance; it's a fervent issue that people feel strongly about. I personally don't know how I could live with the shame if Taiwan was to be allowed independence; I'd much rather die in the war. Your statement that China would rather let Taiwan go than go to nuclear war disqualifies you from making any progress here. Because you think this, that's why everyone here is basically talking to a wall when trying to educate you on the situation.
On the other hand, the US is a country that would not go to lengths for Taiwan, as it sees it as a pawn. If the US were interested in democracy or "freeing" Taiwan, it would have done so decades ago. In the 80's and '90's China was an insect compared to its modern day self. At any day, the US could have acted but it always weighed the benefits and risks of helping Taiwan and China has always provided it with enough incentive in a combination of economic, diplomatic, and militaristic ways, to back off. That's why this issue is still here today and I don't see how in 2017, China will be unable to do at least what it had done for so many years before. Yet, the only permanent incentive for the US to let the issue die and to stop milking this pawn must be an overwhelmingly powerful China, which is where we're headed today.