There’s nothing private about public behavior or any sort of actions that require legal intervention or privilege.
Again, you’re talking about changing public law.
By the way, there are various sorts of legal partnerships, powers of attorney, etc., that can be obtained for most purposes by any two people or any group of people.
RE: your second point. That's fair, but limited. Again, these issues are all state law matters, and accordingly have wildly different results in different states.