Perhaps some of the following will help. The Caddo Light Horse, Captain William B. Denson, was mustered into Confederate service on May 3, 1862, at Shreveport. The company went to Arkansas and served as an escort company for at least one general in that state. It, with Captain L. M. Nutt's Red River Rangers (also from Caddo) were a part of Colonel Robert R. Garland's brigade at Arkansas Post/Fort Hindman and were captured at the surrender of the garrison on January 11, 1863. After the men were paroled from prison and exchanged, they returned to Arkansas and again did escort duty. On November 17, 1863, the company was relieved of that duty and ordered to Shreveport. The following month, the company became Company A of Harrison's Louisiana Cavalry Battalion. Some of the officers and men became the nucleus for Company D of that battalion. Possibly in September 1864 (or as late as January 1865), Harrison's battalion received several new companies and became the 6th Louisiana Cavalry Regiment. The regiment appears to have disbanded at or near Mansfield in May 1865 as it became clear that the Trans-Mississippi Department would be surrendered to Union forces.
The reason that there is no information on Harfield McCormick's release from prison is that he escaped. A dispatch written at Vicksburg on March 27, 1863, states, "Surgeon [H.] McCormick, C. S. Army, escaped from the enemy last night, and joined us." This is in the "Official Records," Volume XXIV, Part 3, page 692. It is unclear if he and other POWs were on a Union vessel of some kind when he made his escape, but I think that probably was the case. He had been released from a hospital in Memphis on March 14. I do not know when or how he returned to the Caddo Light Horse. He was examined by a Medical Board in Shreveport in January 1865 and became assistant surgeon of the 6th Louisiana Cavalry Regiment. McCormick signed his parole in Shreveport on June 15, 1865.
As for McCormick being chief medical officer under Ben Butler, I do not believe that was possible. Butler arrived in New Orleans in early May 1862, and McCormick was already with Captain Denson's company. Butler left New Orleans in December 1862 while McCormick was with his company.