Nah, it's still you who needs to get your history straight.
Tibet was under Qing Dynasty rule from the late 1600's to 1911. You'd be pretty hard pressed to find any history book that says otherwise (while the Qing Imperial court gave the Lamas a lot of self-rule leverage, there was also no question that the Tibetan Lamas and subjects recognized the authority of the Qing imperial rule). And the Republican China's claim to be the immediate successor state to the Qing and its defined territories. Followed by the PRC's claim to be the successor state of the first Chinese republic. Again, that is the basis of the claim - what was China's recognized territory as of the Republican revolution of 1911, and that, included Tibet.
Whether you agree or not, my point is to expand your view horizon a bit and learn that China's territorial claim didn't just sprang into being out of no where in 1950.
And you would also be wrong to claim "Taiwan was never part of mainland Chinese history".
Taiwan was an adjunct colonial territory to China's Fujian province during much of the Ming and Qing dynasty rule, pretty much since the 15th Century (it may have been far off the corner of the East Sea and largely forgetten by the Imperial court in Beijing, but they did administrate it, however haphazardly). The Qing's briefly made Taiwan a full province a few years before ceding Taiwan to Japan after defeat in the Sino-Japanese war of 1895. After Japan's defeat in WW2 in 1945, Taiwan was ... (well, this is where it gets murky, depending on what you want to believe and which historcal document you want to cite - returned or not returned to the Republic of China, is or is not a U.S. trustee, is or is not pending future self-determination), but the fact on the ground is that the Republic of China of 1945 considered Taiwan to be a newly returned province to China and was the effective adminstrative ruler of Taiwan as a province - to this day. So, had the communist People's republic of China ever ruled Taiwan, no. Has the previous Repulbic of China ever ruled Taiwan, yes, and in theory to this day. And if PRC's claim is to be the successor state of the ROC, and the ROC technically doesn't exist anymore, that's the basis of the PRC's claim to Taiwan.
The PRC claim on the Spratleys, believe it or not, is an extension of the PRC's claim on Taiwan. Because the Republic fo China / Taiwan claim the Spratleys (and in fact have small naval and air bases on the larger islands).
I haven't talked about the perscution of minorities (or for that matter, perscution in general, minorities and majorites) because it's not germain to the discussion of the root of China's historical territory claim. Again, you are boxing it off as if China didn't exist before the PRC. What does genocide has to do with, as you said "if China is one country as I claimed" - what? countries that have recognized territory as 'one country" don't have genocide? Sudan? Rawanda? The Jews in Germany? Didn't all these happen in "one country"?
You wanna talk about genocide? Fine, start another topic.
All I have to do is point out the historical and /or geopolitical context of why China is doing (instead of just shout a few evil chicom die, die, die slogans) and I'll be pretty sure to be tagged by the slogan shouter as a commie sympathizer. So it goes, just another day in the demagoguery corner of FR.
For the record, I happened to lean towards pro-Taiwan independence as well as Tibetan independence (go ahead, look it up, I'm sure you have done it by now, probably already disappointed that you can't call me a newbie). I just happen to understand the rationale of China's action - I don't have to agree with it, but i know where they are coming from and the context behind it - and it helps me to know where they are going. But if you prefer to have them as some unfathomable bogeyman, well, have it your way ... :)