I think the best they could hope for would be for China to attack some US-Nato allies in Asia, like Japan or South Korea. China doesn’t really have the capacity to do much more than that. They couldn’t expect China to help them in Europe any more than Hitler expected help from the Japanese in that sphere.
I doubt that China is going to do anything outside the norm. The harm a Western embargo would do the the ‘harmony’ of China would be legendary.
As big a commie as Xi is I doubt that he wants to cripple his own economy to take Taiwan over a point of pride. Were I Xi I’d unilaterally declare Taiwan an independent country and move on. Let some other Chinese leader think about it 50 or 100 years from now.