If they were economic migrants they'd probably have a good reason to believe they'd have a better life in the UK and have jobs lined up or see them available.
Also because the UK has an ageing population, lots of young people would be incredibly good for the economy and the country. We also have shortages in some fields like nursing and caring that could be filled fairly quickly. In the more medium term there's no reason to believe young HKese could do high-skilled jobs.
If they were from Hong Kong there'd be a lot of solidarity with them. They're not going to accept 3 million, but no country is by themselves. The numbers would almost certainly be spread around.
I doubt many Hong Kongese would believe they were at a significant risk of being killed in the US. If they're choosing to leave Hong Kong, the lower murder rates there will not factor into their thinking. Unsurprisingly, murder rates are highest in poorer areas, and given time HKese would get decent jobs and not be exposed to those risks.
Sure, but over how many years? Hong Kong only shares a land border with mainland China. Emigres could travel to Taiwan, Japan, South Korea and South East Asia - maybe even Australia or New Zealand - by boat, but most would need to fly out. It would take quite some time for half a million to leave.
I'm not sure it's easy to say that HKese who can't accept the situation would leave. There are many older people who would not have the desire to leave their homes.
Also, it's impossible to say what would happen with the protests. As I pointed out, there's no reason to believe such large numbers would leave in one go unless Beijing sent in the troops and they started shooting. Assuming a more realistic scenario, the most militant protesters would probably stay on for a while, but more importantly, if Beijing or the HK establishment tried bring in even tougher laws that could trigger new protests.
Hong Kong is overcrowded, but shedding part of the population isn't a long-term solution. The issue is the lack of housing. Moreover, if as you say the people most likely to leave are young, that would lead to worse demographics in the city and a brain drain.
You suggested that "Chinese" (I assume you mean mainlanders) would be ready and willing to move to Hong Kong. I'm not sure that's the case. Most of the young mainland Chinese I've known didn't really rate Hong Kong, thanks in part to negative coverage in the Chinese media about how the city was chaotic, in contrast to the constant positive messages they get about mainland Chinese cities. Sure, sometimes you get mainland Chinese desperate to get into HK, but they're more like the pregnant mothers trying to get better healthcare and benefits/social housing. They're not eyeing up a lifetime of sacrifice and hard work.
Finally, if the plan is to replace young HKese that leave with mainland Chinese, you're going to just wipe out any gains from less demand on housing, therefore keeping Hong Kong overcrowded.