Actually, this
wouldn't butterfly Islam away. Justinian's conquest did not involve Arabia. And Muhammad was born only five years or so after Justinian died, so there is a change Islam could still arise.
Also, you are forgetting the Sassanid threat to the Levant, Anatolia, and Egypt, as well as the religious friction with the Miaphysites and others in the east.
Also, about Justinian's conquest in southern Francia: look how well "Spania" turned out for the Byzantine Empire. Any conquests in Francia will turn out the same way IMO - weak, unstable, and at the mercy of foreign aggression.
Why is this a historical certainty? Christianity
could have spread by trade, you know. Not everything comes at the point of a sword.
For some reason this really bothers me. How can a ship get blown hundreds of miles off course like that?

Were Byzantine ships even built for that kind of sailing? (I realize that the voyage around the Cape travels close to Brazil, but you specifically said the Cote d'Ivoire.
Because the traditional standby of sticking them in monasteries is abandoned for some reason?
Why would the Byzantines use the Atlantic for trading at all, again? As long as they hold Egypt - and without Islam there is no threat to this

rolleyes

- they can trade directly through Asia rather than traveling west.
Just so you know, the term "Norman" has been used historically to apply to
all Vikings, not just those who settled in Normandy. So I highly doubt that the Vikings themselves would be butterflied away. But I think the one thing we agree on is that the Norman state founded in 911 by Rollo will be butterflied away.