To the best of my knowledge, any meetings with Xi will have just begun. The main agenda item is said to be working towards a "truce" in the nascent trade war, with Taiwan policy a possible topic, but not intended to be the main focus.

Yes, the potential for Trump to basically abandon Taiwan is very much there. That would be beyond foolish (even considering the source), but remains a distinct possibility. China wants so very badly to take control of Taiwan's cutting edge chip making industry, and removing a US military response from the scenario would alter China's calculations massively. China's chance of a successful invasion, rather low with the US involved, rises enormously if they're facing Taiwan, Japan, the Philippines, and possibly (but not certainly) South Korea.

Yes, we woudl be stabbing multiple allies in the back by such an action, but I rather doubt that enters into Trump's calculations. The Iran fiasco has our military response options reduced as it is.