Short answers, pulled from the story.
The Army of West Mississippi was a Union army that served in the Western Theater of the American Civil War. It operated under Major General Edward Canby from the 18th of February 1865 to the 3rd of June 1865, making it one of the shorter-lived named Union armies of the war.
Major General Edward Canby commanded the Army of West Mississippi for its entire existence, from the 18th of February 1865 to the 3rd of June 1865. He named the force himself when he organized the field troops of the Military Division of West Mississippi.
The Army of West Mississippi fought the Battle of Spanish Fort and the Battle of Fort Blakeley. Both engagements were part of the Union campaign to capture the defenses around Mobile Bay in the spring of 1865.
The Army of West Mississippi was virtually the same force as the Army of the Gulf under a different name. When General Canby was appointed commander of the Department of the Gulf after the campaign, the troops reverted to the title Army of the Gulf.
After the Red River Campaign ended badly, Nathaniel P. Banks resigned command of the Department of the Gulf. The forces in the region were reorganized under the Military Division of West Mississippi, and General Canby renamed the field troops the Army of West Mississippi when the XIII Corps and XVI Corps were transferred to the new division in 1865.
The Army of West Mississippi was composed of the XIII Corps and the XVI Corps, both transferred to the Military Division of West Mississippi under General Canby in 1865.