The trick is Hong Kong and the NT needs to be given independence before any treaties are signed with the Communist government in Beijing. If Taiwan can remain independent, so can the entire HKG.
I'm sorry, but this is nonsense. Taiwan was an eminently defensible position, and probably still is today, especially when backed by the US navy.
Hong Kong is completely indefensible. Even in 1949, the UK could not have stopped the PLA from taking the city. And your point doesn't address the NT being leased. The British can't make Hong Kong independent on the grounds that the ROC has recognised Britain's claim to the New Territories.
Firstly, the Taiwanese government depends on the legal fiction that it is the rightful administrator of all Chinese territories, even including ones that the PRC has dropped claims to. If the KMT declare that it's 'ceding' the New Territories to Britain then they are massively damaging their desperately needed credibility, to absolutely no gain.
Secondly, even if the Taiwanese go along with this out of some fit of stupidity or madness:
So what? The PRC's interests were served by letting Hong Kong survive as a capitalist enclave, but that was because they knew they were getting the territories back in 1997 and with them Hong Kong itself. If that's no longer true, then there is no reason for the Chinese not to
immediately occupy the city. There are no circumstances- none- in which Britain can prevent this.
As to the Hong Kongers not being asked what they wanted- yes, that's a shame. But the British never extended democracy to Hong Kong. They weren't going to start with the issue of the handover, because what if Hong Kong wants to stay part of Britain? Britain doesn't want that, and can't afford the resulting crisis. What if the Hong Kongers, as you say, want British citizenship? The British government doesn't want to give it to them. Can you imagine the political ramifications of a Tory government relocating millions of non-white people who largely don't speak English? Neither the Tories nor Labour would want to be responsible for it. There was a mad civil service paper that suggested you could relocate HKers to Northern Island, and take twenty seconds to imagine the consequences that would follow
that.
Look, I recognise that Hong Kong's situation is upsetting. I have family there who want a way out. It is a great city that has been treated awfully by the current government and was treated with, at best, aristocratic contempt by the British.
But moral outrage, however justified, does not change the facts that Hong Kong has never,
ever, been in a position to be independent. If you want a democratic and prosperous Hong Kong, you need to find a way to create a democratic and prosperous mainland china.