The natural formation part only counts and if was not above water at high tides then no matter how much concrete and sand they pour in it is not considered land therefore there is no territorial limits to that formation.
PRC is a signatory of UNCLOS and is a willing participant to the treaty so they should know better then to bury it then call it their territory.
The nine dash line goes against UNCLOS since land is required to make any territorial claim to the surrounding waters around it.
I agree with what you said about everything about high tide and natural formation, but for the record, the nine dash line territorial claim was made and "inherited" prior to the existence of UNCLOS in 1982 (the nine dash line was drawn up in the late 40s by Chiang Kai Shek's ROC).
In other words, the actual basis of the nine dash line was never created on the basis of UNCLOS but rather historical reasons of the time, and the question is whether the nine dash line is automatically invalid because of China being signatory to UNCLOS.
As for some of the reclaimed formations, such as Fiery Cross reef recently featured, what might end up happening is China not acknowledging that the reclaimed islands themselves are natural formations, but at the same time not acknowledging them as international territory either. In other words it'll be a piece of large land in legal limbo, but with PRC structures and possibly some PLA aircraft and assets stationed there. The PRC might not even choose to use the reclaimed islands as a means of asserting sovereignty of 12nmi of waters and airspace around the islands, but simply as a large "reef" that happens to be able to support air operations.
In the same way that a US aircraft carrier is a their sovereign territory but doesn't mean the 12nmi of waters around it is US territorial waters, the reclaimed islands/AKA "aircraft carrying reefs" are sovereign PRC territory but PRC may simply not choose to claim 12nmi territorial waters and airspace rights given it would be legally untenable. So instead what may happen is a small military exclusion zone around the aircraft carrying reefs instead.
I think there is some possible evidence the PRC are aware of this: during the encounter with the P-8, the PLAN side stated "military alert zone" rather than territorial airspace.