what are the legal requirements before one go live 100% on a mobile app business
The one you haven't mentioned is taxes. Maybe you lumped it in with "registered a company" since many states make you show proof of the tax steps to register or to get a business bank account.
The step that should have come before any of the ones you mentioned, especially your MVP, is validating your marketing strategy. That is the thing that proves you have a business that will make money and win customers. If you've left out step 1, go back to it now before you launch. It will tell you whether you made mistakes in development and what you need to change before you'll be on a path to success.