A non-negligible part of this coal is burnt in order to produce things exported to North America (among others), therefore by buying less...

Right, non-negligible. And if those mass-consumers of electricity switch to clean energy it would be great. But a lot of dirty energy is used for heating homes in colder regions on an individual basis.

China manufactures things to send to North America.

This is like saying “Nothing really matters due to the millions of people in our countries that also vote making our vote worthless so don’t bother voting”. If we all thought like this the world would be a worse off place.

Sure, if you ignore population size and accumulated past emissions, it's easy to blame China and sleep peacfully.