Duncan wrote: >>> Duncan was never heard from again, although there is no record of his being killed in action or captured. <<<
He might have died of disease. Based upon my 12th Louisiana Infantry research, the men stationed at Fort Pillow in the spring of 1862 suffered badly from a plague of typhoid fever and typhoid pneumonia. Two dozen died out of 1,200 men accounted for there, and dozens more are "missing" because of the loss of muster rolls covering November 1861 through April 1862. Many, perhaps most of these "missing" were discharged for poor health and sent home.
General Villepigue mentions the First Confederate Battalion in a detailed 6 APR 1862 report of conditions that he found upon taking command at Fort Pillow (OR I, Vol. 10, part 2, pages 394-396). A Post Return for Fort Pillow dated 30 APR 1862 (OR I, Vol. 10, part 2, p. 476) states that there were 385 officers and men present for duty in the First Confederate Battalion out of 519 Aggregate Present and 587 Aggregate Present & Absent.
It has been my understanding that "Aggregate Present & Absent" is the total headcount carried on the muster rolls, while "Aggregate Present" is the total present doing some kind of duty. The difference represents 68 men absent sick, on furlough, or detailed away to other locations. Since "Present for Duty" means armed and ready for service, the difference between "Present for Duty" and "Aggregate Present" suggests there were 234 able bodied men without arms present.
Fort Pillow was abandoned in the first days of June 1862 and General Villepigue's command withdrawn to Grenada, Mississippi. On 12 JUN 1862, he reported 320 men present and 111 men absent in the First Confederate Battalion at Grenada (OR I, Vol. 10, part 2, p. 608). The difference between these 431 "present" and "absent" on 12 JUN 1865 at Grenada, and 587 "Aggregate Present and Absent" on 30 APR 1862 at Fort Pillow represents a permanent loss of 156 men to death, discharge (for poor health), or desertion. Age related discharges for veteran volunteers under the terms of the Confederate Conscription Act did not start until July 1862 in these northern Mississippi commands.
Since Duncan McQuagge was not heard from again, it is likely that he died of disease, and perhaps at Fort Pillow. I just made a visit to Fort Pillow and am still actively searching for the burial place(s) of all of the Confederate dead who died of disease iin 1862. I found two of my 12th Louisiana men buried in the city cemetery at Covington, Tennessee. The regiment had marched through Covington from Fort Pillow in early June in order to board steamboats at Randolph to carry them down to Memphis.
Good luck with your search, and keep me posted if you find anything that confirms Duncan's death at Fort Pillow, or eliminates him from the list.