This is what I don't agree with. It seems you were referring to those treaties to show that the world accepted China's argument. But I don't see the evidence of that; at most they were just non-committal.
Of course it matters how good their argument is – if it was stronger than enough countries would be interested in maintaining stable international order to back them up.
>Of course it matters how good their argument is – if it was stronger than enough countries would be interested in maintaining stable international order to back them up.
No international actor wants a regional Asian hegemony except China, there's no argument strong enough to override such interests. And there's no argument strong enough to form consensus among 6 claimants or US desire for Chinese containment. SCS is simply not a dispute that can be solved legally / multilaterally anymore, unless bilaterally under Chinese terms like original UNCLOS accession conditions. In the meantime, China has the industrial capacity change the conditions on the ground. Probably the most expedient method now, hence arguing over claims is not productive. The claims exist with clear historical documentation, for pretense of Chinese enforcement, that's all that matters.
Which is inevitable, even in the 60s when significant countries were fine with Chinese claims, these legalities barely mattered to resolving the disputes since no interested party had the power to enforce anything. There's so much resources and security tied to maritime control in this region that this is only a dispute that can be solved through force. Along the same lines, it doesn't matter what regional or major world players think about Chinese legal arguments if China can enforce it via tonnage. Right now the only viable opposition is USN + FONAPS. But that's kind of status quo US+China is comfortable with, everyone dick waves without touching tips.
The important point in terms of "claims" is where previously plurality parties endorsed Chinese legal nitpicks (hence China has the earliest and strongest claims), now they do not, not because other claimants have better arguments, but because geopolitical interests changed. The quality of the arguments do not matter, nor the laws if we're honest. China explicitly did not submit to UNCLOS arbitration mechanism, so the PCA ruling hack was not to decide on sovereignty but maritime boundary but defining what is / is not features to deprive China (and other countries) of EEZ and territoriality. Right now China "adheres" by limiting weaponization, and drafting other legal arguments to legitimize holdings for more territorial perks. But that's just a stalling tactic until Chinese Navy is large enough to control the region defacto.