What would have to occur for Tibet to become autonomous,or for HHDL to return?
Well, for HHDL returning to Tibet, it would require Chinese leadership that saw wisdom in letting HHDL return as a means of boosting China's image or solidifying ties with India for some purpose. The former is unnecessary for now because their denunciation of him as a separatist has a function in their internal propaganda. The latter at the moment won't happen because both countries are competing for dominance in the region and border disputes are ongoing. India housing HHDL is beneficial for India's image on the global stage (soft power protection).
True autonomy of Tibet would undermine China's plans for resource extraction and their long-term strategic concerns. After half a century of cultural genocide, they won't trust the Tibetans not to start separatist movements. They know Tibetans wouldn't become happy PRC citizens and never challenge Beijing, even if there were democratic elections (which may or may not ever occur). There's already a huge Han Chinese population in Tibet, too, which cannot be told to leave at this point. You have second and third generation Han Chinese in parts of Tibet, too. Lhasa is a Chinese city now. So how would autonomy work? Also, would it be in China's long-term interests? Probably not.
They don't need to be nice to Tibetans to impress anyone. They know money talks, plus the cold hard realities of military power. Nobody is going to pick a fight with them or even start a trade war over Tibet, and they know it. It might have been possible in the 60s as the CIA intervention back then would suggest, but that's long past. China and the USA are business partners and the elites have a lot of common interests now.
It isn't like Ladakh in India which from the start had cultural and religious freedoms. The India state gave them education, healthcare, electricity and roads. They got rich pretty quick with foreign tourists, too. The military bases there are not a huge problem for Ladakhis, both the Muslims and Buddhists. Local forms of governance are recognized and encouraged, but the state of Jammu and Kashmir calls the shots on overarching matters. But the Ladakhis seem fine with this and are content with being in the Republic of India. They get sufficient autonomy in their daily lives.
Incidentally, HHDL has a summer residence outside of Leh. I visited it when he wasn't there.
My sense from your comments (they ring true) is that a free Tibet is now only an aspiration, and not a likely one.
I don't see how it could happen unless the Chinese state collapsed and all the Han Chinese in Tibet left. But then they probably feel Tibet is their home, so they might not be so quick to leave.
All things considered if the Chinese state collapses and something like a civil war erupts, the suffering resulting from that might be greater than the cultural genocide committed against Tibetans over half a century.
Some people here might think it would be just retribution and an opportune moment for Tibetans to reclaim their land, but that ignores the fact that a billion people in China suddenly losing a stable state is a lot larger a problem than Tibet.
We hear a lot about the plight of Tibetans, but hardly anyone in Tibetan Buddhism recognizes the plight of average Chinese people, both now and historically. They went through hell and back under Mao. A lot more Chinese than Tibetans died under the boots of Mao and his sidekicks. Cultural genocide was exercised against Chinese culture, too.
Basically, we're always talking about Tibetans, but ignore the fact that a billion Chinese people need to be considered as well. It might not seem warranted including them in consideration of an independent Tibet, but in reality their welfare at this point in time
actually depends to an extent on the military deterrence gained through China's control of Xinjiang and Tibet. Nobody is going to invade China from the west, but if you're going to call the shots in Asia and ensure you have access to resources you might not otherwise have (and are now necessary for supporting a billion people and their raised standard of living), then you need to possess hard power.
If China was to walk out of Tibet tomorrow the well-being of a billion or more Chinese people would be compromised. That's why there's no political will to do it. Zero.
It isn't just, but then geo-politics and the realities of the world are not fair.