Blitzo, there was never a problem of FON in the South China Sea. Your remarks "China simply has the largest claim and is able to enforce it most easily" highlights that the PLA-N is capable of enforcing sanctions/barring entry into the South China Sea, according to the dictates of the Chinese Government. That is the current problem which encourages the US and its allied countries to patrol these waters regardless of China's claims. I repeat that it's time to negotiate peacefully under UN/International law, and not for China to bulldoze its way in this matter.
FON is never a problem for anyone, if problems and conflict and underlying disputes are resolved.
If China and the US go to war and the US has control over the SCS then FON for Chinese shipping will be a problem. If China and Japan go to war and China has control over SCS then FON for Chinese shipping will be a problem.
In other words, to fully resolve the FON problem, one needs to build mutual trust and look at the underlying issues.
The fact that the US is claiming that China is a threat to FON is because China has differing interests and have areas of potential conflict with the US and some of its allies in the region, therefore believe greater Chinese military presence could threaten the shipping of its allies. Of course the fact also exists that the US has the immense capability of threatening Chinese FON in the area as well.
Therefore for the purposes of discussion, the subject of "Freedom of Navigation" is a misleading one, because every country with a coast is interested in Freedom of Navigation for their own shipping and the shipping of its (economic or military) partners, and saying a country is a "threat" to Freedom of Navigation ignores the reasons and the particular situations for why a country may feel the need to threaten Freedom of Navigation of some countries.
I propose, that instead of saying "country X is threatening freedom of navigation" which is a general phrase lacking clarity or rationale for why country X may threaten freedom of navigation, and lacking overall context, better say "country X is increasing the ability to protect its own shipping and (economic and/or military) partners, and increasing the ability to threaten the shipping of country Y (a potential adversary of country X)".
This way, the phrase is more neutral, objective and can be applied to any country in a fair and equal way.