It doesn't really matter what the POD is, maybe the Crusaders conquer Egypt, maybe the Fatimids don't decline or maybe the Makurian rulers simply decide not to raid Egypt.
So, regardless of the POD, what if the Nubian Christian kingdoms of Makuria and Alodia survived? How would their continued existence affect the region? Would they have friendly relations with Portugal, in the 15th, 16th and 17th centuries, like Ethiopia did? Could they escape colonization, like Ethiopia did?