What are you fucking talking about. Hong Kong has always been part of China. The imperialist monarchical tyranny of Great Britain forcibly occupied Hong Kong and separated it from Chinese governance on a temporary contractual basis. Once that contract was up, Hong Kong was once again part of China but instead of completely overturning everything in Hong Kong, the Chinese government established a way of working called "One Country, Two Systems" that is making the transition away from the dynastic monarchy of unapologetic genocidal racists easier for the people of Hong Kong.

The fact that the youth of Hong Kong decided to protest part of this process is a function of British propaganda. The protestors elders were not in support of the protests and some even threw their kids out of their homes for their participation. Meanwhile, the protesters in Hong Kong were incredibly violent for a prolonged period of time and the response from the Chinese government was incredibly sedate, patient, and considerate, especially compared to literally anything the USA has done domestically when people protest for the rights of black people.

Your narrative is fantastical with little to no basis in reality.

A war between Taiwan and China would absolutely become a World War.

Reunification can be peaceful like it was with Hong Kong.