Use US-purchased Watch 5 Cellular with a Chinese carrier, anyone?

No.

iPhone Cellular Carriers that support the Apple Watch, which models, the radio frequencies each model uses, and the country where they offer service:

https://www.apple.com/watch/cellular

The North American Cellular Apple Watch (Canada, U.S., Mexico, Puerto Rico) will not work in any other country.

And a cellular Apple Watch from another country will not work in the North American area.

Different radio in the North American models vs the rest of the world.

Using a regional radio makes for a smaller radio chip, and uses less power, then using a full spectrum all frequencies chips.

When they get the fusion reactor on your wrist, then maybe they can power the full spectrum radio. Or maybe when the chip vendors reduce the size and power requirements for the radio. But then they will change the standard, to say G5, which is a power hog on any frequency, and once they have that tamed, they will come up with G6, then G7, so maybe the fusion reactor is more realistic 😀

The Series 5 listed the ability to call emergency services world wide. (911 in the U.S.). That is not the same thing.

Series 5 Fall Detection and Emergency Services on Cellular Apple Watch Series 5 with no cellular contract

https://support.apple.com/HT206983

https://www.apple.com/watchos/feature-availability/#branded-international-emergency

https://support.apple.com/HT208944