My word the southern reaches of the Chinese claim are extremely grabby! Is this just similar to haggling in a bazaar for an item listed at $30, which you personally value at $20 so you open with a lowballed offer of $10?
You're implying ASEAN members all act in unison -- they do not. They fight amongst each-other almost as much as they do with China.
And while they fight amongst each-other, China voices support for one while undermining that support in another fashion. Pitting them against each-other is common.
You're being generous. China is annexing territory that belongs to Vietnam, Malaysia, Indonesia, and the Philippines. The same thing that Russia is doing in Ukraine. It's an act of war, the issue is that the world is very afraid of China (both militarily and economically).
No, it's a territorial claim - so far. It's not yet an act of war. Trying to enforce the claim will be an act of war.
Building the islands... well, everybody else who claims that area also seems to be building there. That could, arguably, be considered an act of war, but so far everybody's doing it and we do not yet have a war.
And while they fight amongst each-other, China voices support for one while undermining that support in another fashion. Pitting them against each-other is common.