As to why the PRC chooses to block Taiwan in every diplomatic engagement, the answer is simply that the CCP does not want any other nation to aknowledge in the least Taiwan as a sovreign independent political entity. It may seem petty to block Taiwan from practically everything no matter how insignificant, but the thinking in Beijing goes is that the blackout must be total for it to be effective.
As for the issue of Chinese Nationalism. I will have to answer that question with the default reply that it is designed to provide legitimacy for the CCP now that it has abandoned Marxism-Leninism. For people who think that the PRC is going to somehow fragment along ethnic lines, that is pretty much simple wishful thinking by anti-China stealth imperialists. A line of thinking highly popular with Taiwanese separatists, Japanese rightists, and the occassional beetlenut addled Westerner who thinks fancies himself a Sinologist. If the PRC were to fragment, it would be more likely along provincial or political lines, with certain cliques or cadres within the former PRC carving out local power niches for themselves. That China will fragment into Tibet, Xinjiang, Manchuria (which doesn’t even exist anymore no matter how much people use the term, now it is simply Liaoning, Jilin, and Heilongjiang), and the always ludicrous “south” is simply the wistful and quaint dementia of sinophobes.
As for the Manchus not being Chinese, I am afraid that is no longer the case. Originally the Manchu’s were regarded as foreigners, even up until the Republican Revolution which used this to provoke nationalist sentiments to overthrow the empire. However, after that to promote national unity, such rhetoric was downplayed by the early Republicans. The communists continued to downplay ethnic divides, with Mao Zhedong even writing against Han Chauvinism in his desire to promote one Chinese identity. The fact of the matter is, this campaign to promote national over ethnic awareness has been successful. Today, the political and social reality is that manchus are Chinese, if not han-chinese. That they were considered a foreign conqueror then is irrelevant. They are no longer foreign now.
As to the notion that everyone in the PRC is some kind of mindless communist drone, I am afraid you should exercise more your allegedly “free” intellect.
“I have the greatest admiration for your propaganda. Propaganda in the West is carried out by experts who have had the best training in the world – in the field of advertizing – and have mastered the techniques with exceptional proficiency … Yours are subtle and persuasive; ours are crude and obvious … I think that the fundamental difference between our worlds, with respect to propaganda, is quite simple. You tend to believe yours … and we tend to disbelieve ours.”