You're conflating so many questions here that it's barely worth responding, but here goes:
1. Whether a people deserve autonomy, or statehood, need have nothing to do with whether they have had autonomy, or statehood, in the past.
2. Whether Tibet should have statehood, normatively speaking, has nothing to do with its geopolitical consequences for China.
3. Whether a people deserve autonomy, or statehood, has nothing to do with the (often hypocritical) behavior of Western leaders and states.
4. If you want to discuss this question, you need to consider the normative justifications typically construed for ethnic and/or national autonomy and self-rule. Having arrived at a position of what you take to be justified, you must then consider the practical/political consequences.
5. In some respects, Ukraine/Russia serves as a nice point of comparison to Tibet/China.
No, not at all. There is no historic precedent for irredentism in the case of Tibet, as opposed to say Taiwan.
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This is rather false. Tibet was unquestionably part of China under the Qing dynasty. At no time at all after 1720 has Tibet ever been recognized as a de jure independent state by the international community.
Whether or not Tibet should have "independence" comes down to Cold War geopolitics and the desire by the west to dominant China. There are 3 million people in Tibet, in mountainous highlands overlooking 1 billion Chinese people. The west doesn't acctually care about those 3 million people, they care about having geopolitical leverage against China. China would never simply grant independence to Tibet, because the next day there would be NATO bases there, and China would never cease to be at risk of an invasion which would be difficult to repel.
Tibetans, unlike Taiwanese, are a separate ethnic group and different culture from Han Chinese, and on those terms should be allowed to practice cultural autonomy as a nation. But that's