you may want to take a look at
and see which countries control what. The vast majority of islands in Spratleys are little rocks of little value. China, at great economic cost, has expanded 4 of its controlled islands into real military stations. I would argue that's all China needs to dominate south China Sea. The other little rocks that Vietnam and Phillippines have are of little value. The next thing it needs is to have military bases closer to Australia and Malacca straits. It should be pretty obvious to this thread why China would find these things beneficial.
So, all China needs is to keep its expanded islands floating since they could very easily sink from this ongoing crisis we have called the climate change. Work with Phillipines, Malaysia and Indonesia to divide up the rocks and then work out some kind of model in dividing up the gas drilling rights. I would say there is much greater value in settling these issues than arguing over how many rocks they have. Once you do that, try to sell as much weapons to Indonesia and Malaysia as possible so that they are in China's oribts. Continue to invest in these countries and influence them.
As for your other point, the reality is that Chinese people control the Indonesian economy which has caused a lot of jealousy in Indonesian population. What better way to help those people out than bringing Indonesia into China's orbit.
Everyone needs friends. US claims all of the Americas as its background through the Monroe doctrine. How do you expect China to secure it's trade and energy route without more allies and bases around the world? With what? US can cut china's oil trade off by just moving a couple of its carriers groups over. If you think China should be a superpower, then it cannot trust that when global resources are being crunched up, America will not take it all for itself and its allies.