Of the two chronographers, the founder of Christian chronography, Sextus Julius Africanus, whose Chronicle came down to A.D. 217 or A.D. 221, transmits the Epitome in a more accurate form; while Eusebius, whose work extends to A.D. 326, is responsible for unwarranted alterations to the original text of Manetho. (Introduction, xvi-xvii.)The passages I cited were from Eusebius' Latin edition of Manetho. It is possible (though unprovable either way) that the material about Ham and Aegyptus is an interpolation by Eusebius. I really hope it isn't, but there's the possibility it is. (The detail about Mestraïm is most likely authentic, since it appears in a number of places in Manetho's work outside of Eusebius' edition.)
Still, even if it is an interpolation, it's coming from an early Christian source not far removed from Manetho. It wouldn't be as convincing evidence for a Hellenistic Book of Abraham, but it shouldn't be ruled out entirely either.
Of course, if it is an interpolation by Eusebius, then my comments about Manetho being exposed to the Hebrew Bible are to be disregarded.
So, like I said, I hope this material is original to Manetho. But in the mean time . . . caveat lector.