A friend of mine who I had emailed for help on this question came up with part of the answer. In One of Cleburne's Command: The Civil War Reminiscences and Diary of Capt. Samuel T. Foster, Granbury's Texas Brigade, CSA, editor Norman Brown footnotes Foster's story of McPherson's death. He identifies the Sgt. Major of the 17th/18th Texas as James Mathis, and quotes Mathis essentially as I did in my initial post. In his account, Foster identifies Private Henry H. Cowan of his own Co. H, 18th Texas, as the man who fired the fatal shot. I still have not discovered the name of the soldier from Co. C, 17th Texas, who was also credited with killing the general.

Brown also cites several other sources on McPherson's death:
Battles and Sketches of the Army of Tennessee by Bromfield Ridley
Reminiscences of the Boys in Gray, ed. Mamie Yeary
Cleburne and His Command by Irving A. Buck

If anone out there has access to one or more of these books, I would appreciate a lookup on that unknown private from the 17th Texas. (I have already found the references to McPherson in the Confederate Veteran and didn't get any help there)

Thanks for all your replies to my query.