You mean the KMT that set up after they were kicked out of the mainland and took Taiwan right? They also claim all of (outer) Mongolia and basically whatever the Qing dynasty had when it fell. There CPC doesn’t claim Outer Mongolia and its borders are mostly settled with a few conflicts in India and the SCS.
You are referring to the de jure claims, which Taiwan only maintains due to pressure from the PRC. If Taiwan officially cedes these claims, the PRC would respond with economic sanctions, because the PRC wants Taiwan to continue to pretend it is another faction of the same country still fighting the civil war and not an independent country.
Yes. If Taiwan decides to stop being the ROC and instead wants to be the Republic of Taiwan, that would be considered a huge escalation by the PRC. Right now, they are just two sides of a civil war, which the PRC can accept, but succession would be viewed way different.
Luckily, Taiwan can just continue with the status quo and the PRC will probably not invade it, especially when the Americans are hanging out in the Ryukyus so close by to the island.
just for additional context the Taiwan people have no dreams of occupying China by force or any means, other than the dissolution of CCP on its own but even then most Taiwanese wanted to stay as economically independent as possible, since they can fare off better on their own as evident in this current pandemic, restraining ties with China is a good measure