I am not a lawyer and I don't even live in the USA, so my opinion is really worthless... but I'm going to write it anyway

The POS of the L14 resorts have not been modified, the SSR POS I've read doesn't even mention a multi site POS. So any change made to multi site POS is irrelevant.
Someone buying resale assumes the same rights as the original owner. So someone who owns at the L14 and RIV (or reflections) will be interested in doing a lawsuit to remove the restrictions.
I don't think someone buying RIV resale would have any ground for a lawsuit.
At the time, someone said: since all the current owners have been grandfathered, for them RIV has actually joined the Club like all the other resorts. I don't remember the exact wording, but the POS doesn't say that future resorts must join with the exact same rules, but with in a
substantially equivalent way. So for current owners, RIV joined in a substantially equivalent way.
I bet this is the interpretation that DVC will go with if challenged.
It doesn't account for the loss of value of contracts, but that would be difficult to demonstrate (the first half of this year contracts at the L14 went actually up) and a resale value is not guaranteed anyway.
So I wouldn't be sure a lawsuit would win, but still it is possible.