You correctly outlined the brief service history of the 54th Tennessee Regiment. Tennesseans in the Civil War (p 281) mentions consolidation of this regiment took place in April 1862. Col George H Nixon's commission dates from April 7, 1862, which confirms other regimental records showing that Nixon's 48th Tennessee organized on that date. See Capt Stephen C Burch, Quartermaster, who stated that the two regiments (Voorhies 48th and the 54th Tennessee) were consolidated on Apr 7, 1862.
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Requisitions issued by several company officers formerly of the 54th Tennessee were issued at Corinth MS on April 13 and April 16, 1862. These are --

Capt Thomas R Hughes, Co A (Co F, Nixon's 48th Tennessee)
Capt Lewis C Miller, Co B (Co G, Nixon's 48th Tennessee)
Capt James N Curry, Co C (Co H, Nixon's 48th Tennessee)
Capt John D Ives, Co D? (Co I, Nixon's 48th Tennessee)
Capt James M White, Co E (Co K, Nixon's 48th Tennessee)
Capt Jasper N Hollis, Co F (Co F, Nixon's 48th Tennessee)
1st Lt James H Merriman, Co H (Co F, Nixon's 48th Tennessee)

Did not find requisitions for Capt Andrew J McMackin's Co "G", Capt William T Childress' Company or Capt McKinney Dooley's Company, which became Co E, Nixon's 48th Tennessee.

William B Cunningham, 4th Sgt of Capt Ives' Company, was captured at Shiloh on Apr 7, 1862. Although Nixon's 48th Tennessee was not assigned to a brigade, it must have been part of the garrison of Corinth MS. Accounts by Alabama troops assigned to garrison duty who could hear the fighting from their camp at Corinth mentions the impulse to go forward and become involved. Perhaps element of this regiment went forward with other members of the garrison. At any rate, it is clear that Cunningham was wounded at Shiloh, and naturally gave the name of his regiment at it was earlier the same day.

While revewing record of the 54th Tennessee, I noted that William C Rowland, Co "G", 54th Tennessee Regt, was captured in Federal uniform at Shiloh. On April 11, 1862, Rowland was sentenced to be shot.