On the historical
Their is no doubt that a warrior/prophet arose in the late 500's early 600's and preached a fierce mono-theism amongst the arabian tribes and launched them on a mission to conquer the lands that were now vunerable because of the great Persian war the Romans had just fought, lands they believed were their right through Ishmael. The actual details of the conquest and the interpretation of the foundational story seems to be mostly taken from authentic sources but has been reverse engineered by Abdul Malik during his Califate. Mostly the geography, which was a somewhat fluid thing in the minds of people of that time. The Holy Place is where the holy thing is... move the thing, the geography moves, but it is still The Holy Place in the mind of the believer so it never moved as well.
Heres one that just popped into my head. The Roaman Emperor Elagabalus was also the high priest of Elgabal and custodian of the Black stone of Emesa. Somewhere during the crisis of the third century, probably during Zenobia and Aurelians struggles in the late 200, the stone, or what was left of it, seems to have been taken to the Nabataen stronghold of Petra. An ancient well know city on the crossroads of major trade routes and a place of temples and pilgrimage from the arab peninsular for centuries. A place that fits the desciptions given of mecca/bekka far better than the place we have today.
Dan Gibson has shown that every Moslem prayer Niche was pointed at Petra until the time of Abdul Maliks Caliphate, from India to Africa to Persia, this is solid stuff. The Muslims used to explain it by saying the early niches were pointed at Jerusalem and this was changed when Malik built the Dome of the Rock and pointed its niche at what is today Mecca .But modern analysis has shown these early niches pointed a couple of degrees south of Jerusalem right at Petra. The civil war of Maliks, seems to have been when the stone was taken by the rebels to present day mecca and when victorious Malik liked the idea of the religious centre another 1000 miles south and out of his way.
It does not seem reasonable to believe that Mecca, which is only mentioned once by name in the koran,was an already 2000 year old trading city, not on the trade route by 100 miles, and holy site yet not recorded on a single map or mentioned in a single source until the 740's. It is a much better fit to place the origin of Islam amongst the arabian tribes on the periphery of the Nabataen kingdom 1000 miles north of present day mecca. All the actual koranic stories fit this better. it is the Haddiths that explain the foundation story as we have it today that point further south and they all are after proper Imperial control in the early 700's. Actually the majority are from the 800's onwars.
If the foundational story of Islam is actually totally different than the accepted tradition does that make a difference to whether they are a divine manifestation of a continuing message.
As an aside i think that is why the christian story is more believable. By the time Imperial political types got their hands on the the religion there were already so many copies of the different books spread over so many different nations and languages and so much commentary already done by the previous generations that they could not rewrite or do any substantial editing. I bet Constantine wished he could have edited the books at Nicaea but they were stuck with established stuff that went back centuries.
I'm not sure where to go with this. Its based on the opinions and conclusions of historians who are controversial and have been subject to criticism. It would be intellectually dishonest of me to say that I could discount these theories as I'm not an historian and I haven't considered the evidence one way or the other. Its always difficult with religious history being able to weight up the facts. You ask if this alternative historical narrative would make a difference as to whether or not Muhammad was a Divine Messenger? It could do. Would it make a difference to you if mainstream history in regards early Islam was more definitively proven correct? I suspect not.
What we do have is the Quran. The historicity looks solid, specially compared to the Christian scriptures. Does the Quran constitute evidence of a Divine Messenger comparable to moses or David in the Hebrew Bible? I believe it does.