Thank you for filling in more of the details regarding the Union soldiers and the bushwhackers referred to in the documents. I'll keep digging...I certainly never expected to learn this much. The fact that Keith and Clark were 1st Regiment Provisional EMM matches the info on the folder in his CSR entitled Thomas Clark Wm Keith vs George Darne. It seems to be signed by J B Douglas cmdg 1st Prov Regt EMM in Mexico.
Interestingly, someone with whom I exchange genealogical info has suggested that his body might have been returned to Callaway County by train for burial since the Darne and Blackburn (his wife's maiden name) families were both prominent in the county and would have had the resources to do it and at the time of his death he was a civilian, although in violation of his oath.
Were the trains able to run with any regularity from St. Louis to Callaway County in August of 1863? My limited knowledge suggests that it might have been possible.