You have done a yoeman's work on the 7th MS and a 'tip o' the kepi' to you for all you've done.
Here's my 2-cents worth as well. I believe several companies (I and K) of the 1st MS Lt Art'y served with various elements of the 4th MS Cav in SW Mississippi and the 'elbow' of LA during the Port Hudson campaign in particular. They were known as the Jefferson Flying Artillery (Darden's) and Withers' Company.
My research of the 8th MS indicates they wintered in northeast MS around Corinth in late December 1864 following the disasters at Franklin and Nashville. They were put on troop trains there and sent to 'jine up' with Johnston in NC for a final showdown with Sherman. As you well know, the AoT had been decimated and only around 5,000 of that army actually made it to NC. I believe they were mostly infantry and may or may not have included artillery units since they had to be pulled by horses/mules and very few existed in MS or elsewhere at that time of the War. In summary, I'm not sure any elements of the 1st MS Lt Arty ever made it to NC in March/April of 1865.