1.b The protesters' demands are all about changing the behavior of the local government. They have no reason to split from China unless the mainland government intervenes to prevent that change. As a bonus, consider that the current Hong Kong government is only in power due to the election system with functional constituencies put in place by the British to secure their grip on the city. In a way, China simply replaced Britain as the colonial power rather than freeing Hong Kong from colonialism. (On the other hand, this argument can backfire since Hong Kong would be even worse off if all former British institutions were replaced by their mainland equivalents.)

2. If all protesters were violent rioters, they'd have conquered Hong Kong by now. But the majority are peaceful and have even less power to stop the rioters than the police; whose indiscriminate use of tear gas and rubber bullets has hurt people who were either participating peacefully or just happened to be in the area. That's why they want an independent investigation of the police.

3. The economic situation may make them more likely to protest, but what they choose to protest is still something they care about.

Consider the protests that erupted at Nanjing School of Applied Technology in April [1], at Beijing Normal University's Zhuhai branch in June [2] or in Wuhan's Xinzhou district also in June [3]. Were those all for economic reasons? Also ask whether they were aware of these or any other protests.

[1] https://www.epochtimes.com/gb/19/4/28/n11220048.htm

[2] http://www.epochtimes.com/gb/19/6/11/n11315690.htm

[3] https://www.epochtimes.com/gb/19/7/4/n11363038.htm

(I'm linking to Epoch Times articles, but in each case you can find raw footage on YouTube searching for "南应", "北师珠", "武汉阳逻")

Finally, you should know that there just as you are here asking for help arguing with your Chinese friends, so are Chinese people asking on Zhihu for help arguing with their Western friends.