I don't know that would have been a deciding factor. The first thing Lee did after the 7 days was to reorganize his army, primarily by advancing less senior officers based on their performance in the field. He would have gone with whoever he thought best able to do the job, regardless of seniority.
It would have been. When Lee needed two corps commanders following Jackson's death, Hill's date of rank for Lieutenant General was one day after Ewell's so he remained the junior of the two. Had Jackson survived, Ewell would have gotten the third corps.