Various points:
Naval blockade worked against Cuba. That said, Cuba is a small island, whileChina is the largest country on Earth. Go figure.
China, a Third World country? My god, better read that than being blind.
Patriots: ask the IDF ( the Israelians ) how good they are. They suck, plain and simple.
Carriers: China may have outdated ships, subs and planes, but they have a ****load of them. If you align ONE more target than what the carrier fleet can handle, it's a sure hit. I don't think there's any pb for the chinese doing that.
Guerilla. As proven in Irak nowadays, the US army is unable to deal with that. And chinese have been used to doing guerilla ( against the japanese ) and proved they could do very well.
That's for defense. As for China invading another country? In their current military state, it's as possible as USA invading them. They can't, they just don't have the logistic to support a prolonged, distant war as they could on their own soil.
I read lots of funny things about WW3 in the 50 following years, chinas turning into the next big threat, etc, but get a gripe people, taht ain't gonna happen. Same goes for their exploding economy: they'll face the same thing every country faced at that point: growing health, growing understanding of the populace about their own rights, growing social requests, etc, which will slow down the whole thing w/o any external influence.