My understanding is that SSUS is on the National Register of Historic place. This only recognizes a structure or area as being “worthy of preservation.” It doesn’t place and particular restrictions on the owners. The entire neighborhood I live in is on the National Register, and we only have to abide by our city and county codes. We can build, tear down, and modify as we see fit within building codes.
I’m also interested in the assertion that NCL doesn’t own the vessel outright. Everything I can find, including government registries, list the owners as Maritime Investment Inc., with the same physical address as NCL’s Miami headquarters. I would very much appreciate any documentation you have that indicates otherwise.