Maybe $50 is low, but it's just a first idea. The cleaning is going to be in a very limited area, because you're only going to allow people in very limited places. And they're paying the port costs anyway, so that shouldn't be a consideration.
What permission do you need? They're not actually going to be cruising. They're basically Fishlips, except they're only at their location for the day. They'd have to meet CDC requirements for restaurants, and maybe for hotels, but not for actual cruising.
I'm sorry to be dense, but I don't understand what being non-US flagged has to do with offering a tour and a meal. I don't mean that to be snarky. I honestly don't understand the connection that you're making.
I have a limited understanding of all of this, and I know more about other cruise lines than Disney, so take this all with a grain of salt.
If they were to operate as a restaurant and/or hotel at Port Canaveral rather than as a cruise ship, they should have to abide by regulations for U.S. businesses, which are entirely different than the ones they operate under normally. You wouldn't be able to use the normal cruise staff because they're not U.S. citizens, and the Customs and Border Patrol lawsuit in 2015ish says you can't even do a cruise-to-nowhere with non-U.S. staff due to immigration concerns. So you'd have to hire and train a U.S. staff and pay them accordingly. Beyond labor laws, the U.S. has stricter laws about food service, liquor licenses, pollution regulation, etc. that cruise ships do not abide by. There might also be local zoning or other regulations. My understanding also is that the costs are very different for parking an empty ship at a port vs. operating a ship at a port. Plus the security implications, especially if the area is a working port for other types of vessels.
Further, though, there is the tax situation. Overall, most cruise lines pay very, very little in U.S. taxes. That's why when there was talk about bailouts for the travel industries, cruise ships are a bit of a sticky situation. I assume if a ship were to operate a tour or as a dining venue in a U.S. port, there would be significant tax implications.
In the past, some cruise lines have allowed day-guests (particularly for weddings), so it may not out of the realm of possibility. Also the Queen Mary operates/d as a floating hotel out in Long Beach; however, she is moored permanently and is also on the National Register of Historic Places. (Also she's closed right now because covid and California.)