Most of the complicated dispute whether Tibet belongs to China or not lies in the unaware and shortsighted politics of the early Qing Dynasty which started 1644 and ended 1911 by the revolutionary forces under Dr. Sun Yat-Sen.
In historical papers, it was written, that China has its Garrisons in Tibet in the early 18th century. it was said that Tibet and China was bounded (China gave them military protection and Tibet gave them goods etc.)
Back in the days the Emperor in Beijing never thought that not putting a clear and formal status of the Chinese-Tibetan relationship would become a problem in the future or better say in the present (same problem with the Diayu-Islands and Taiwan).
Neither Tibet nor Taiwan were under direct rule of the Chinese Emperor, but they were always bounded and under Beijing's influence.
Then the Opium War and the invasion of the 8 foreign nations (Russia, USA, Germany, France, UK, Japan, Portugal, and ??) ripped China apart forcing it to be cut into pieces, losing territories such like HongKong, Macao, Singapore, Tsingtao, Shanghai, and also Tibet etc. (Taiwan was lost in 1895 after Japan took it by force).
Even during the regime of the KMT from 1911 - 1949, China was not united again, although China got back Taiwan in 1945. Then the civil war in China caused another new setting of China's national border. Losing Taiwan to the escaping KMT but seizing Tibet again. But the masskillings in Tibet did not happen in the seizing but was caused by the resistance in 1960's. Most Chinese believe that it was not ignited by Tibetans themselves but by Western Anti-China factions who wanted China to fall apart again.
Tibet was conquered or re-conquered by the Communits of China and annexed by them.
Tibet is since then officially not an independent and sovereign country anymore. Free-Tibet activists might think different but that is the fact. You cannot claim a region as yours again if you have lost it at war or by annexion. Otherwise Germany could claim Preussen (now Poland) and Elsass-Lothringen (now France) back and American Natives would like to throw all White,Black and Yellow skinned Americans out of their continent.
As what you said before about the change for Tibet and other parts of China:
Yes, the Tibetans were most stubborn not to accept formal changes during the early years of the PRC. Today, China's government has also granted people in Tibet to live out their religion, just as the muslims in Xinjiang.
The fools who still believe China is massacring Tibetans everyday are the best audience for any Anti-China groups, including the FLG and the Free-Tibet activists.
I remember when Premier Zhu replied to an Free-Tibet activist: "You think that we still kill people in Tibet and opress them like you claim? Then maybe I should pay you the flight ticket to Tibet and please see for yourself what the reality is today!"
During the 1960's and 1970's millions of Tibetans were killed because of resistance and persecution. But how many
Han-Chinese died at the same period? It was much much more. Tibetans were not killed because they were Tibetans, but because they were part of the whole Chinese society which was forced to follow the Cultural Revolution.
The Cultural Revolution was a nightmarish time for China (yes, including Tibet) and almost cost the life of the later chairman Deng Xiaoping.
Even todays chairman Hu Jintao lost his father, who was executed because he was a capitalist.
Dalai Lama himself has adressed to the public that he does not want people to push China with Anti-China actions but just to pray that todays and tomorrows leadership in China can improve life for Tibetans.
I think he finally understands that there is no way China would let go Tibet becoming independent unlike Outer Mongolia, which was lost because China was too weak to hold it.