At the core of China's claim seems to be that it sees the current situation as a continuation of the relationship between Tibet and the Ching dynasty. That doesn't seem to be correct though as Tibet was not part of Ching "sovereignty" and the Dalia Lama was not a subject of the CHing dynasty. So we see a typical Western concept of "sovereignty" applied to a political and cultural situation that doesn't really fit.
Another approach is a rights-based approach. It is interesting to see that Lenin (and Woodrow Wilson) argued for self-determination. The USSR and early communist China recognised the right of secession. This was later removed from the Chinese constitution (1930s). The right has nevertheless been reiterated over time in several treaties signed and in some cases ratified by China; e.g. the UN Charter, the ICCPR and ICESCR. China at least pays lip service to the following idea of self-determination:
Article 1
1. All peoples have the right of self-determination. By virtue of that right they freely determine their political status and freely pursue their economic, social and cultural development.
2. All peoples may, for their own ends, freely dispose of their natural wealth and resources without prejudice to any obligations arising out of international economic co-operation, based upon the principle of mutual benefit, and international law. In no case may a people be deprived of its own means of subsistence.
3. The States Parties to the present Covenant, including those having responsibility for the administration of Non-Self-Governing and Trust Territories, shall promote the realization of the right of self-determination, and shall respect that right, in conformity with the provisions of the Charter of the United Nations. — ICESCR
Obviously, taking into account how, for instance, Africa was carved up by colonial powers this raised some issues as people worried about the defragmentation of existing states. Prevalent view for the exercise of self-determination has become that self-determination only applies to: entire populations living in independent states, or entire populations of territories yet to receive independence, or territories under foreign military occupation.
This is a restrictive definition excluding groups of people we'd usually refer to as... well "peoples". It excludes Kurds and various African tribes for instance. We're also confronted with the fact that most states recognise the
de jure or
de facto sovereignty of China over Tibet, which is a strong argument for the current situation.
The subjection of peoples to alien subjugation, domination and exploitation constitutes a denial of fundamental human rights, is contrary to the Charter of the United Nations and is an impediment to the promotion of world peace and co-operation. — United Nations General Assembly Declaration on the Granting of Independence to Colonial Countries and Peoples
This principle was applied to:
- the Russian invasion of Afghanistan
- the Vietnamese invasion of Cambodia
- the occupation of Arab territories by Israel
- Estonia, Latvia and Lithuania by the former Soviet Union
- the Grenada by the United States
- the Iraqi occupation of Iraq
On the basis of the above, how are we supposed to interpret the independence of Kosovo? We see that the prevalent view doesn't apply
unless you'd accept the Albanian Kosovars were under alien subjugation, domination and exploitation. In other words, despite sharing the same country for a long time the Albanian Kosovars were culturally sufficiently different from the Serbs to be considered "alien" from each other resulting in subjugation, domination and exploitation under Milosevic's rule.
Finally, self-determination need not mean independence. Autonomy is key. The Dalai Lama has repeatedly argued for atuonomy instead of independence and this could be supported by the Tibetans (I don't know if they do).
Based on the Kosovo example, I'd argue Tibetans are a separate people from the Chinese with a sufficiently different culture than the Chinese to be considered separate. China will argue primarily that Tibet has been part of China since the 1600s. However, it appears that the political and social relationship between Tibet and the Ching dynasty and Tibet and China since 1950 are different things. This is by definition reflected by the treaty ( Agreement of the Central People's Government and the Local Government of Tibet on Measures for the Peaceful Liberation of Tibet, or the Seventeen Point Agreement for the Peaceful Liberation of Tibet) signed by Tibet and China which refers to the Tibetan people as separate from the Chinese but as "returning to the family" of motherland China. It is not clear either how you liberate people within your own country or why a treaty would be necessary to establish a situation that already existed according to the Chinese. That's only possible if Tibet was indeed autonomous from China.
Based on the above, I think a continuation of the relationship between the Ching dynasty and Tibet would be one where Tibet was autonomous. Chinese refusal to accept such autonomy is a "subjugation, domination and exploitation" of the Tibetan people and as such the right for self-determination can be invoked by Tibetans and should be pursued by the international community.