Dear Aineo
Maybe I should ask you this: Why would Muhammad borrow from the heretical Christians and the Jews? Jews and Arabs are not exactly the best of friends. Why would he antagonised the Arabs by telling them Jewish stories and lifting up the image of Mary (a Jewess) against his own wife Khadijah?
Similarly, how do you think he would be able to attract the Christians if he were to tell them stories which Christians think are not true and not in their Bible?
Although there are Christians and Jews living in Medina, it is not a sufficient reason to say that he borrowed the stories from them.
Muhammad also gave the Coptic Church certain guarantees that Muslim's have ignored and violated since his death
I really could not comprehend what you are trying to say
salam