I am not sure why you are labouring on US motives because what is core is China has make certain claims and so the issue is not the US but China but somehow you seem in my view trying to make it a US issue.
I'm not making it a US issue, I'm just trying to look at the motivations for each side's actions. It is as much a China issue as it is a US issue.
Unfortunately China's claims and the rather ambiguous nature and scope of the claim has implications including on FON.
Since the end of WW2, the world has enjoyed a period of peace (relatively) because there is some form of law and order. A legal framework in which the majority abides by it helps to maintain a certain degree of peace that we enjoy. So legality is nor misleading as you stated but rather under-pining the environment. The issue as is, is not even about a legal resolution because no one even knows China's legal premise of the claim. In other words, we don't even have a starting point that anyone knows outside of China.
Trying to paint a geostrategic picture into the equation is unnecessary, distracting and not helpful if we want to keep it simple.
This isn't a matter of challenging "law" (as in the entire global order), but rather a particular law or a particular set of laws -- out of many laws.
Any law that underlies the world order (or any national order) is built atop geopolitical/political and strategic interests and agreements.
And nations will interpret, accept, enforce or challenge individual laws with differing degrees of resolve depending now how a certain law may cause material and physical consequences for them.
Those consequences could be anything from slightly cutting down a trade deficit, to the potential projection of power over a significant body of water that a significant trade flows through
In other words, I am looking at the motives and material consequences of countries when they are looking to challenge or enforce a particular law, because the effect of any law on any individual, town, state or nation will range from positive to neutral to negative, and those differing entities will respond to a law depending on how it affects them and how much it affects them.
If you really want to frame this from a legal frame of view rather than primarily as a geopolitical one, that's fine, but further interrogation into the roots of one's
willingness to enforce or dispute a legal position or an interpretation of a law, inevitably leads one to see the material and physical (i.e.: strategic) consequences for either side which underlie this situation.
And the goal shouldn't be to "keep things simple". The goal should be to "keep things accurate and comprehensive". We should try to seek a clear but wide spanning and insightful interrogation of situations as is practically possible... because most situations including this one is anything but "simple".
The LOS convention is clear on the nature of the 12 nm zone and EEZ. The problem is some countries want to impute on it rights and restrictions which are clearly silent and not mentioned in the convention. The LOS convention was developed to prevent and settle disputes. There are obviously still areas that not all countries see eye to eye. I think we are jumping ahead on this because, until China clarifies its claims, we actually do not know on what basis it is making it - whether according to LOSC or some other form.
The point I was making was that countries would naturally try to interpret laws to their own material advantage and/or to avoid vulnerabilities.