Artificially-constructed islands don't count for expanding EEZ per UNCLOS. What China appears to be doing is attempting a mixture of trying to dilute the facts so as to increase ambiguity and developing "facts on the ground" (cf. the Israeli settlement patterns in Palestinian territory) so that any grand compromise hews more to their favor.
I think the last thing China wants is a multilateral treaty: the more people that are involved in a treaty, the less powerful China's negotiating time is. China has made repeated angry noises against proposals to treat the issue as a single regional issue rather than a collection of bilateral disputes. The disputes are also a large factor into why all the other south-east Asian countries (including especially Vietnam!) are trying to cozy up to the US and get the protection of the US Navy.
Some of them are natural islands that have been artificially expanded to support a military presence.
The 12 NM territorial limit for a natural island certainly allows the power owning it to build barracks, airfields, and artillery batteries within that limit. And that prevents other countries from claiming or disputing the island.
I think the last thing China wants is a multilateral treaty: the more people that are involved in a treaty, the less powerful China's negotiating time is. China has made repeated angry noises against proposals to treat the issue as a single regional issue rather than a collection of bilateral disputes. The disputes are also a large factor into why all the other south-east Asian countries (including especially Vietnam!) are trying to cozy up to the US and get the protection of the US Navy.