Decent analysis, but the fundamental problem with that is you are trying to retrospective apply UNCLOS to historical claims made centuries before UNCLOS and the very notion of territorial waters and EEZs were a thing. That is the central reason why the 9 dash line doesn’t conform to modern norms.
These claims were grandfathered in when China joined UNCLOS, which China did by exercising standard provisions within UNCLOS created precisely for such examples.
China’s current policy is deliberate strategic ambiguity in terms of what its historical claims means. That is to give itself maximum leverage to try to settle disputes with other claimants, but also to allow itself room to upgrade its claims should the need arises.
The SCS issue was essentially a minor non-issue until the US tried to turn it into a stranglehold against Chinese sea base trade, since that is the only realistic place on earth where anyone could actually try to cut Chinese shipping lanes.
Do it further away at Malacca strait and you have the mother of all administrative burdens trying to sift out ships heading towards China from the rest of the world and most likely strangle world trade before you could strangle Chinese shipping. Do it closer and you get annihilated by the PLA.
The US wanted its cat paws of the Philippines and Vietnam to grab islands and build military bases that the US could then ‘lease’ to dominate the region. Worst case they fight the PLAN far from mainland Chinese air cover and only one carrier and win easily.
What they did not even dream of was that China could build artificial islands to the scale and at the speed that it did.
Now not only will the PLAN have land based air cover from the island bases, it also have comprehensive long range sensor networks and bases for its vast fleets of type 022 FACs and 056A corvettes.
China has won the SCS. Western FON patrols are just bad looser antics that speaks of a lack of real options on their part rather than a demonstration of strength. It’s like street thugs walking in front of police stations giving the stink eye. They might think they look menacing, but they aren’t intimidating anyone sitting in those police stations since they know they can easily turn those thugs into Swiss cheese any time they please. That’s also why we are hearing less and less about the SCS, and why US propaganda attacks have since moved on to HK, Xinjiang and now Taiwan. It’s just a catalog of failures for them.