If I were to venture a guess about the ankh, back in the '70s a lot of Black Pride movements adopted Egyptian symbolism. Their reasoning tied into the idea that white people back in the day taught that Africans were uncivilized brutes, basically, and that the African continent produced no great civilizations (totally wrong, but it was commonly said). Thus back in times of slavery, this was used to justify keeping slaves in bondage (they would say that if the slaves were freed, they would just "revert" to their uncivilized African ways).
So long story short, when the Black Pride movement happened in the '70s, black people looked to the Egyptian civilization as proof that Africa produced a great civilization - something that African Americans could be proud of and point to as a great ancient achievement. It would make sense to me that a predominantly black Christian church might use a cross mixed with an ankh (and the ankh has its own spiritual significance that could be adopted as well).
That's just a guess though.