I enjoyed reading your post very much. I too had an ancestor, one of my great-grandfathers who took the oath and tried to enlist. He had spent 6 months at Camp Douglas April 1862 to September 1862 after he was wounded and captured at Shiloh. He returned to his regiment and stayed until captured again at Missionary Ridge where he lost his right eye. On November 11, 1864 he took the oath and tried to enlist to go west and fight indians but was rejected due to having one eye. That must have been a horrible decision for him to make because his grandfather was a full blood Choctaw indian so I feel sure he knew how the white man treated his grandfather's people.
If you ever get a chance to read Lonnie Speers' Portals to Hell: Military Prisons of the Civil War do so and you will see why so many took the oath. In cases like his these men were returned into the prison where vengeful former compatriots often tried to get at them. However there was some groups who "reenlisted" these men in the Confederate army. Of course that was never an official fact but at least it gave some the opportunity to repent in the eyes of their friends. Be warned though.... reading that book is not what one would call a "pleasant read". But their stories need to be told and I found that with enough tissues you can read the whole book.