We´re talking about an upstart religion among the backwater (sorry) tribes of Arabia here. It should only be in very rare and LUCKY cases that it manages to unify the penninsula, much less defeat the two major powers of the world and become a world religion. It should not happen often
What if we added several 'alternate' invasions such as a West African invasion, a early Turkic invasion from Turkestan and even a major slavic power forming a 'Slavic Empire' of sorts to conquer Europe? This way the map could change but it wouldn't always be Islam that does the changing
I gave this suggestion before, If we are limiting Islam this much we need to have something else to effect Europe from the outside besides the Mongols
Also, that statement makes forming Islam and making it as it is historically as likely as the AI reforming Rome and taking back its historical borders in vanilla. Something that -actually- happened in real life shoule be fairly more likely then the restoration of Rome or a Tengri Holy Roman Empire