Supply is a a bit complex as you see:
Ideal case, you have a land route to a main source, making your cities as a distribution center, and generating 10, if this is not available, than each city and other source generate some amount by itself (8 for the largest cities, 5 for big ones, 3 for the smallest settlements), but this is maxed to their current lvl, which reduced if changed hands and if siege happens, which can go down to zero.
Also occupied cities can never go over 8, so even from the start you generate 8 in chine, and the chinese generate 10, before encircling, cut routes and destruction comes into play.
For siege, two units needs to be next to the settlement, encircling and siting one not enough, in that case, the city (nanning) will provide 5 supply forever, but two or more, and nanning will go down one per turn, giving you the city in time, but after that, it will stay at destroyed, and will grow with one per turn too, so this also limits your actions from that city later, unless other source can help out.
No, two source do not add together, just gives you more safety to get even that small amount.
If the Chinese gar attacked, than it is a bad luck for you mostly, or something else, like your own gar is very low on morale.
The most important part for that area, never allow the chinese to cut the railways on the coast, those lines what supply your forces, if cut, you will have insane trouble (and if you play with the chinese, your goal is to cut those lines.)