How is that annual 50k USD monitored/enforced now? If I have multiple bank accounts, will I be prevented from withdrawing a combined 50k from overseas ATMs?

It's connected with the citizen number and it's applied to personal currency exchange only.

I think you may ask for a higher limit but usually you just use family accounts.

Paying with a multi-currency credit card has nothing to do with this limit.

So, the bank will report these currency exchanges (including overseas ATM withdrawals from an RMB account) to some central authority like PBOC, who will then combine it with info from other banks, and thereby know if I've breached the annual limit?

There is an real-time system shared by all banks. I can see my remaining quota immediately updated whenever I made an exchange.

In your example, if you have RMB in your China bank account and withdraw RMB overseas, these are no currency exchanges at all. It's not relevant to the $50k exchange cap.