If I am remembering the Quran correctly, don't they also refer to Adam and Jesus as a prophets? Mohammed was the last prophet, not the only prophet. I think you are fine. In Islam, they also have Aniconism- aka no images of god of the prophets(which is why they get upset about images depicting Mohammed). If you make your prophets teaching focused on himself and his image and his worshippers holding his image sacred, then I think it would be pretty obvious your religion is not Islam. Even without that I think most would understand the potential of a false prophet so if you show the readers hints of his two sided nature I think you are fine. Just because Islam is the major religion today that uses the term Prophet commonly in reference to Mohammed, the term is not solely Islamic. For other terms: Oracle, Visionary, Herald, Guru, Spokesman, Disciple
