I have a 1984 reprint of the "Register of Confederate Soldiers and Sailors Who Died in Federal Prisons and Military Hospitals in the North" compiled in the Office of the Commissioner for Marking Graves of Confederate Dead, U. S. War Department, 1912. The list for Point Lookout shows that Private James M. ELLIS, Company M, 7th Confederate Cavalry died at Point Lookout on 17 AUG 1864. It also lists a Private James N. ELLIS, Company E, 26th North Carolina Infantry who died at Point Lookout on 23 JAN 1864.
Organized as Claiborne’s Partisan Rangers on 10 SEP 1862, later designated the 7th Confederate Cavalry, the regiment was commanded by Colonel William C. Claiborne, seconded by Lieutenant Colonel Thomas D. Claiborne. Several companies were swapped in and out during the life of this unit. The 7th Confederate Cavalry was broken up in July 1864. Companies A thru E and Company L were sent to the 10th Georgia Cavalry. Companies F, G, H(2), I, and M were sent to the 16th Battalion, North Carolina Cavalry. This suggests to me the origins of these companies were in Georgia and North Carolina respectively. There are no apparent Mississippi connections to the 7th Confederate Cavalry regiment.
The Compiled Military Service Records for the 7th Confederate Cavalry shows that Private James M. ELLIS was enrolled in Company G, 7th Confederate Cavalry at Mocksville, North Carolina on 3 SEP 1862. He was a resident of Davie County, North Carolina where Mocksville is located, was 34 years of age, and a farmer by occupation. At the beginning of 1863, he was transferred to Company M, 7th Confederate Cavalry. He was captured at Morris Mills, Virginia near the James River on 14 APR 1864 and sent to the Point Lookout Military Prison. He died there on 17 AUG 1864.
Private James N. ELLIS, a resident of Chatham County, North Carolina was enrolled at Cartersville, North Carolina on 28 MAY 1861 to serve in Company E, 26th North Carolina Infantry. He was promoted to 2nd Sergeant in April 1862. He was wounded and captured at Gettysburg, sent to Fort McHenry and forwarded from there to Fort Delaware arriving in the time window of 7/12 JUL 1863. He was transferred to Point Lookout from Fort Delaware on 18 OCT 1863 and died at Point Lookout on 23 JAN 1864.
Neither of these two Confederate soldiers was from Mississippi, or Alabama. Taking the Point Lookout records off the table should help refocus your research.