And DeSantis can't (or wouldn't) do anything about this? At least early on, he was on-board with Trump (or appeared to be). You would think he would push through some bill that would allow the state of FL to relax the rules for Trump.

My understanding is that these restrictions are in part the creation of a private agreement/deed restriction between Trump (or whatever entity actually owns MaL) and the City. The State can't disturb those. Partially they're the result of the City's exercise of its historic preservation powers, but those are delegated down to the cities and counties from the state under our general Home Rule statute. Since the state doesn't make the call, there really isn't a mechanism for them to authorize a one-off exception.

There have been various small proposals to pre-empt local government historic regulation in very limited circumstances - usually to require local governments to allow coastal developments to harden their homes against sea-level rise or hurricane dangers. There was a bill in the last session to allow owners to disregard local historic designations if their home was seaward of the Coastal Construction Control Line - a line of regulation that ensures development standards to resist storm surge and whatnot. I don't recall whether it passed or not, but it wouldn't affect MaL.

Out of curiosity...what's so special about that mansion? Did someone famous own it before?

It was owned by Marjorie Post - who owned General Foods and was at one time the wealthiest woman in the U.S. - and E.F. Hutton (yes, that E.F. Hutton). The mansion was built in the 1920's, which makes it pretty old by Florida standards - and it was designed by a very prominent local architect. Those two factors make it historic, by our lights.

Post donated it to the National Park service upon her death. The National Park service gave it back to her foundation after a while, because it was just too expensive and difficult to maintain. Trump bought it four years later to be his residence, only to switch gears and try to develop the land instead. Palm Beach was having none of that, given the history of the property, so we find ourselves in the present situation.