No they wouldn't, TW doesn't have the ordnance or ability to deliver said ordnance to structurally damage a gravity dam, especially one size of three gorges. They're much better off hitting PRC coastal nuclear (something that worries PRC planners), either way, it's suicide by war crime.
Yes. If Ukraine executes Russian POWs, or firebombs Moscow, it's still a war crime even if Russia invaded with the goal of genociding them (yes, saying an ethnicity doesn't really exist and they're just confused Russians, kidnapping children to resettle elsewhere, and forcefully assimilating at gunpoint everyone in the occupied territories is genocide).
War crimes are absolute, there's no "if you weren't first, you get one free".
Is it still a war crime AFTER the CCP invades with the goal of completely replacing the Taiwanese government?