Stephen,

You are challenging us to think "outside our box" on perhaps the greatest spiritual challenge of our time. Bravo!

While I am not sure yet how to answer your question about accepting into Adventist fellowship a person who believes Muhammed was a prophet, I must allow for that possibility because it is our personal relationship with God that gives us salvation and God does not ask me to measure another person's relationship with Him, just to support and nurture it. Since our relationship with God should be progressive, I must allow both for that person's relationship with God to grow past old beliefs as their relationship with Jesus grows. I also must accept that their experience can teach me things that will nurture my relationship with God.