The depiction of Jesus in the garden of Gethsemane, in great distress and praying that the cup pass from him, is one that it is hard to imagine being invented by the later church, after they had made sense of the cross as the decisive salvific event in human history. Would they invent Jesus asking for that not to occur? James McGrathThe question isn't whether the later church might invent the story. The question is whether anyone might invent the story.
We know that the Luke had no qualms about changing details in Mark in order to tone down the distress that Jesus experiences in the face of death. For example, he changes Jesus' last words from “My God, my God, why have you forsaken me?” to "Father, into your hands I commit my spirit." Is there any reason to think that Mark would had some qualms about altering the stories that he heard in order make whatever theological points that he wanted to make?
If we accept that the evangelists felt free to alter the stories, then we have to accept that anything in the stories is there because it served the author's rhetorical purposes. Mark could have thought it important to detail Jesus' human frailties in order to emphasize the transformative effects of the resurrection. It doesn't mean that Mark's story can't be true, but I don't see how we can base our conclusion on the notion that he would have been reluctant to change the stories as they came to him.