How can I start a business on an EB-5 EAD?

You can engage in your own business on stand-alone EAD. To maintain H-1B employment terms however, you should continue work for your H-1B employer until I-485 is approved. You may appoint a manager for your own business to run it for you and be an owner and receive distributions.

You can use your EAD to start a business but then you are moving off your H-1B. if you close your business, and leave on an advance parole, you may be able to reenter on that H-1B, provided you are only working for the H-1B employer at that time.

Should not be a problem.

You can start a business/invest in a company while on an H-1B visa. However, you cannot work for your company/draw a salary (as you can only be employed by your H-1B employer). If you formally use your EAD to work for your own company, you will fall out of H-1B status. You can hire people to run the day-to-day business for you - you can profit off of your equity in your company (as an investment) - you cannot draw salary or work fulltime while on your H-1B visa, however.

If your objective is to maintain valid H-1B status while your AOS is pending, you can only work for your H-1B petition. Whether launching a business and any relevant activities constitute "work" is a factual matter that you should discuss with experienced counsel. Maybe get a concurrent H-1B petition approved by the new company?

If you have a work permit, you can do any work you wish to do including starting your own business.