Hi HM - You may have missed the thread about Tom Holland's book/documentary which iirc espouses the idea that the Prophet did not exist. I can't locate the threrad right now, (don't remember which section) but someone else may know. Anyway it is Tom Holland and the book is 'Shadow of the Sword'.
While I haven't read the book yet, I have heard some interviews with the author. I have also seen the documentary Islam: The Untold Story. Again, I'm not against the idea that Muhammad did not exist, I just tend to think that the evidence that is put forward in support of that proposition is rather weak.
Firstly, a lot of it stems not from any evidence to the contrary, but rather from an alleged lack of evidence to support the existence of Muhammad. While as a skeptic, I do believe that the burden of proof lies upon the party making the assertion, I also believe that there has to be some sort of counter narrative--an explanation as how else things could have got to where they are-- given if you are going to dismiss a historical claim. The more that I can poke holes in your counter narrative, the less likely I am to believe it.
So, for example, I haven't heard of any historians doubting the existence of Yazid bin Muawiyah bin Abi Sufyan, nor the fact that he was the third Caliph of the Umayad dynasty. I've heard no one doubting that Yazid was at war with Hussein and Ibn Zubair after his father's death.
Now, Yazid clearly claimed to be the son of Mu'awiyah, and the grandson of Abu Sufyan. The seerah attests to the fact that an Abu Sufyan existed, who opposed Muhammad until the conquest of Makkah, and who had a son named Muawiyah who went on to become the governor of Syria and the Caliph after Muhammad's death. That's the Umayyad story and the seerah backs it up.
Hussein clearly claimed to be the son of Ali, and the grandson of Muhammad. Even as bitter enemies, the stories of these two men corroborate each other and concur logically with the seerah. How do we account for this if the character of Muhammad was not founded in reality?
Another argument I've heard is the lack of place names in the Qur'an linking it with Makkah. That is simply not true. The names Makkah, Yathrib, Arafaat, badr, and Hunayn all appear in the Qur'an, and these places are well known in the Hijaz. Furthermore, the context in which these places are mentioned implies that the listeners were very familiar with where they were and must have known the stories behind why they were being mentioned (battles, treaties, pilgrimage sites, etc.) The lack of detail makes it seem unlikely to me that these place names were inserted later on.
As for the origin of the Qur'an, I have no reservations accepting that Muhammad copied his "revelations" from earlier sources, both oral and written. Again, this does not conflict with the narrative of the seerah. Muhammad was not from "the middle of the desert," but was a city dweller. Even if we accept that the importance of Makkah was exaggerated, there were definitely Christian and Jewish Arabs across the Arabian Peninsula, from the Fertile Crescent to the lands of Yemen. As a merchant, trading Yemeni Jewish gold and Frankincense to be used in the churches of Byzantium, for instance, it is unlikely that Muhammad would not have come into contact with other faiths. Muhammad had 40 years before he started claiming prophet hood to spend a week or two here and a month or two there listening in on Jewish folklore and Christian legends. He was a loner, he was intrigued by the world around him. It doesn't seem strange to me that he would have done so.
I don't think we need to dismiss him from the pages of history to dismiss the faith he preached as false.