Due to Earth curvature Ground Radar at best can detect something in the air at 350km and sea target at 50km. One can relied on satellite but everyone has access to satellites surveillance even tiny states as they can just paid airbus for access  and it can only pass an area once every few hours. By that time the ship is already gone but Chinese stolen island can't move and has a fixed GPS coordinates.

Buying missiles & jet is easy but The challenge is ISTAR.  Chinese equipment can only talk to another Chinese equipment but other countries asset assuming they get NATO standard one are capable of cooperative engagement.

If a shooting start with tongsan & US. Then other ASEAN state can just kepochi sending their MaLE,MPA,awac and claim to be neutral but at the back feed American with intel.

If the American blockage the straits of Malacca, Persian gulf, Panama & Suez canal then there's nothing the Chinese can do about it.

It's the Chinese that had their back against the wall rather than the American.