I know that the tribes have no freedom now; you could not do anything like a charter city without changing the Federal laws. The laws need to be changed.
Some places would refuse to sell land to outsiders, but some would. The tribal governments should be able to, without Federal oversight, choose to either manage land communally, or assign property rights to individuals.
Yes to both questions. This is what I see as the most politically feasible way to make a charter city with actual regulatory independence happen in the USA.
Based mostly on reading Matt Levine, I suspect that in practice, if the SEC wanted to go after a business registered in the CDEZ, they would.