I wouldn't assume they were 2 band enfields without some sort of documentation. The Enfield carbine, rifle (P-56) and P53 (3 band) musket were used but you can't be sure who was carrying what without a primary source. I have read about calvary carrying the P-53. I have also seen one that was cut off with a hacksaw in front of the middle band and ID to a CS horse soldier. In 1864 the CS Calvary was doing a lot of fighting dismounted. No telling what they were using. IMO, Confederates were using more variations of the Enfields (such as the bar on band) than is generally thought.

I don't know what a Mississippi carbine would be. The M-1841 was a rifle and only came in one length. It is possible he may have been referring to one of the CS mfg. carbines.

It is unusual that the ammunition would be described as .54 cal carbine and .54 cal MIssissippi cartridges. They should be the same cartridge.

Hope this helps a bit.