2 - Is there still a "Brown-headed nuthatch colony just north of the research centre" ? where is "the research centre"?
3 - Is there still a "Snail Kite roost at the Miccosoukee rest area"
Which part of Florida will you be targeting for Brown-headed Nuthatch? I do not know of a 'research center'. I have seen them in various areas, including Three Lakes Wildlife Management Area and the pineland area of Everglades National Park (reintroduced population).
I don't know of a Snail Kite 'roost' at the rest stop along Tamiami Trail, as you said, but they can be regularly observed from the abandoned airboat concession stand. I also tend to see them in winter late in the afternoon/evening on Anhinga Trail in Everglades National Park amongst all the other returning waders that come to roost (including American Bittern, storks, herons, egrets, etc).
I can't comment about Ding Darling, but I can recommend Tigertail Beach -- lots of shorebirds if you are willing to walk.
Carlos