See The Full Captain
and https://bastard-bird.deviantart.com/gallery/57256157/Marvel-Family
Captain Marvel and Billy Batson were not the same person. In the Fawcett comics as they were originally published it was made mighty clear that Billy and Cap were pretty much spacial anchors for each other and swapped places when struck by the magic lightning. Unfortunately, every time Captain Marvel was brought back after the '50s the Captain was just an older, bigger body for the mind of Billy. It doesn't help much that Captain Marvel Jr was Freddy Freeman with a cape and Mary Marvel was Mary Batson just more powerful. Even the Lieutenants Marvel w