Finally, I don't understand why the Arabs needed Islam to unify. The Mongols didn't, after all. Its just that any area they conquered would not be inflenced culturally by them.
well, the Mongols got a Great Man {of History} to unite them....Islam got both a Great Man and a New Religion in a single package.
but minus Islam?
well, Mohammed would still be able to get the people of Yathrib (Medina) behind him - he settled a deep dispute in their city, and in OTL, they converted as a gesture of gratitude...in this ATL, would they follow him, sans conversion?
it's a good question, though -- in their eagerness to get rid of the Byzantine yoke, would Egypt and Syria accept the overarching authority of a Great Man, knowing that Byzantium would still be there in a generation - and the Great Man's accomplishment (unified Arabia) might not survive his death*?
* = like with Alexander of Macedon. with Temujin, the Great Man's accomplishments continued for a few generations more.