"Ismael, unfortunately (from the Moslem point of view) was displaced from his status as First Born by Isaac because his mother was only a servant and not a true wife as was Sarah."
Which is interesting, because biologically, he's still Abraham's first-born.
You're exactly right. But Ishmael was not the child of the promise. That was Isaac.
The LORD told Abraham and Sarah that they will have a child. Only Isaac fulfilled that promise because he was sired in the union of Abraham and Sarah.
Abraham sired Ishmael but that was only of the flesh.
God also slighted the first born in the case of Easau and Jacob.