Hmmm, maybe we get some way for the Ibadi to be especially close with some branches of Christianity, so they both head down to East Africa around the same time? Though all the slavery stuff would put a damper on things. Apparently some tribes, I think in West Africa, converted to Islam to prevent being enslaved but it worked about as well as some of the tribes who converted to Christianity. Slavers only use religion as an excuse for getting profits. Maybe something with the St. Thomas Christians? Though we would need a much older POD, as they had evolved into a caste and didn’t try to actually convert people, to my knowledge. I had some Indians of that group at a church I used to go to andnthey mentoined How once one of the Muslim conquerors was slaughtering all the Christians they came across, though they didn’t get all the way down Kerala or the tip of India. Maybe find a way to make Christians a seafaring group. Though you will need the Christians to be a good distance into the interior of Africa to survive, unless the coastal areas become a strong state or confederation of cities working together against raids by Somalis, Arabs, etc.