1. Shenzhen doing better than HK is in no way an indicator that HK needs China to interfere politically, or an indicator that Hong Kong's previous economic success was due to Chinese 'help'. Keep in mind HK is also the 2nd largest trading partner China has. This isn't a one way street like you're making it out to be, and with the US trade war this is only going to become more obvious as the US-China trade war continues. For once China needs to admit it was wrong, it can't lose both it's major trading 'allies' due to it's major focus on being an production based country. When I say the UK/west did more I mean they gave the opportunity to those who fled China, China obviously didn't give that opportuinty and to say it was China that 'made HK what it is' is either an incredibly narrow-minded view ora disingenuous point for the same reason you wouldn't claim Germany is what it is today because of their defeat in WW2, hopefully you understand what I mean by this.
2. Not even the same protest. Also, the Reuter's way of counting is unreliable because it only shows how many people cross a certain point (admittedly high traffic area), however this was a city wide protest in both the 1 million, 2 million, and the protest you are referring to.
3. Influence in what way? Maybe I am misinterpretting what you mean, but most are upset over the influence China is trying to put in the political (China maintains a strong and overbearing relationship with HK's CE along with the CE having to be practically approved by Xi before they could be leader of HK) and education system (blatant propaganda, and the teaching of Mandarin rather than Cantonese) along with some occaisionsal exercising of brute force on political dissidents (see the book store incident).
Why would China let go? Although I understand one of the Chinese evils is speratism (and that's probably an ideological difference that I very much disagree on) they look bad, and in the modern world international perception is incredibly important. Countries don't want to trade with tyrannical countries, and people in HK are willing to look bad if they think China will look worse, they tried to be peaceful and didn't get anything they wanted (Umbrella movement), but they start being violent and suddenly they're getting what they want. Nobody is complaining about the violence the blacks committed in the US during the civil rights movement, nobody complains about the Allies bombing Germany, few complain about nuking Japan twice, because it ultimately made a positive impact on the world, whether it be ending WW2, crippling the Nazis, or having civil rights for former slaves and others of colour. The ends, more often than not, justify the means.
There is a problem in regards to criminals in Mainland, Macau, Hong Kong, and Taiwan, anyone can see that, but they way the extradition treaty was put forward along with the already strong fear/hate of China, due to reasons I've already said, in HK was outright stupid and to think any other reaction was going to happen was an oversight to say the least.