What changes in Islam, and history if Meccans just support Muhammad from the start, rather than oposing and fighting him?
There's quite too much butterflies to deal with that. I would think that it would have an equivalent situation to IOTL.Would Islam spread as far as it did?
I don't think so : granted, Mecca wasn't the city Yatrib was, but Hejaz as a whole had a real potential, critically with former Arabic states declining.Or would it be restricted because of it and die out eventualy as a tribal religion?
I don't see that happening : the Meccan hegemony over Arabia is gonna be as much fought as Yatrib/Meccan was IOTL during Ridda Wars. You'll have an imposition of Islam over submitted or allied tribes quite quickly, and that would be formative on how Islam would spread.If the Meccans had embraced Islam from the start, I can see the subsequent expansion of Islam being more peaceful, much like Christianity had spread 600 years earlier.
I don't think so : granted, Mecca wasn't the city Yatrib was, but Hejaz as a whole had a real potential, critically with former Arabic states declining.
Furthermore, you'll get rid of the dispute between Mohagerins and Ansarians that poisoned up the Islamic succession of Muhammad.
On the short run, it could make Islam a little bit stronger it was IOTL
Not by the VIIth century. Mecca was notably poorer, less important than Yatrib, that was a commercial city existing since centuries, when Mecca made it mostly as pilgrimage center.Mecca was stronger and more populous than Yathrib.
That's not what events seems to show : there's many occurences where Yathrib clans did as they wanted and against the will of Muhammad. The aformentioned Mohagerins/Ansarians (basically native Medinits and Meccans that followed Muhammad in Yatrib) frictions that didn't disappeared depsite his orders (see the succession, and how was elected Abu Bacr on this).But that is not the issue. In Yathrib Muhammad had absolute authority.