On this FL thing, you need to exercise caution.
What is the Constitutionality of the RNC dictating to a State when it can hold it’s primary/caucuses and how it allocates those delegates?
If the RNC is determined to have the authority to do this, then they have the authority to completely ignore every caucus/primary and select delegates by their own choosing in a manner that suits them. The RNC doesn’t like the slate of delegates sent to them by the states? Fine, they go and change them up in a manner that suits them.
I don’t believe the RNC has this kind of authority (thankfully) and that any state can hold it’s contest and allocate its delegates in a manner it sees fit. Personally, the states should file a lawsuit against the RNC because Rule 15 appears to violate equal protection and states rights. No one has ever done this though because the penalty is not severe (1/2 delegate slash isn’t all that, if there were all 100 delegates in FL, all it would do is raise the required number of delegates from 1,144 to 1,194.
If it were a government branch you would be correct, but it's not.
Political parties are private organizations, and courts have always held that they should be able to choose their candidates as they see fit. Primary elections are creatures of state law, but parties can usually opt out of them and nominate by other means. A state political party is free to opt out of their national party’s choice, but that’s only happened twice (1948 and 1960, when Dixiecrat dissatisfaction with the national nominee led to Elector slates in favor of Strom Thurmond and Harry Flood Byrd, respectively).