I looked up that parcel in the county GIS the other day and it just seems crazy that anyone would think it is buildable. It also seems crazy to try and pretend that it should be under federal jurisdiction and that county rules shouldn't apply. I'm no lawyer, but inholding land sits in the county and private ownership predates the park.
From the map it looks like like most of that area is stream bed--it is a triangular parcel with only the "point" being inland.
Attachment 476330
I know the GIS lines aren't 100% accurate, but I don't see how you could put a house there while still meeting any setback requirements and not causing issue with a stream in a freaking national park. The neighboring parcels are much larger and aren't developed either (though they may be government owned). They are all categorized as "Residential Property" but...where's it gonna go?
Don't you have access to an MLS or something that will tell you who handled the deal if it was listed?
Unless it was dirt cheap (because it was useless), I can't imagine trying to buy a parcel like that without talking to lawyers and basically having all of my plans pre-approved by all parties who might even have a slight interest. If they'd really been coming here forever and had family or friends who owned another inholding property, they really should have known better.
edit: lol, well I guess I waited too long to post this and you went and posted the same photo!