Terry,
Another interesting case? Well, let's narrow the clues first, and see if they point to anything.

First, there were two Bohanan households in the Osage Township just east of Lebanon in east-central Laclede County in the 1860 census, although the heads of families were listed as Enoch and Lewis, Jr. There was also a William Bohanan household in Gasconade Township of southeast Laclede County, too. But, you said you were looking for a John Bohanan, didn't you?

Second, maybe your source was wrong about Bohanan's first name. I used Stanley, Wilson, and Wilson's 1983 "Death Records From Missouri Newspapers, January 1861-December 1865," on page 16 which quoted the "Daily Missouri Republican" of St. Louis for 5 December 1861 telling about Lewis Bohannan "of Laclede Co." who died 3 December 1861 "at the Military Hospital" at age 40. Many times when POW's got sick enough, their Yankee jailers in St. Louis sent them to "the Military Hospital" to hopefully get well. This obviously didn't end well for Lewis Bohanan, but now you know the correct name of your source, and it matches the Lewis Bohanan, Jr. of Osage Township mentioned in the 1860 census above.

Third, this narrows our choice of mill sites to Osage Township of east-central Laclede County. Looking a my mapbook, I see the name "Osage" for the township name refers not to the large Osage River about a county to the north, but to the Osage Fork of the Gasconade River which appears on my map to be the biggest drain in that township. That's where I would look for your mill site. I took the liberty of consulting online http://shs.umsystem.edu/manuscripts/ramsay/ramsay_laclede.html which mentions five different pioneer mills which may have been up during the Civil War: Ballinger's on Mill Creek, Farris' on the Osage Fork, Mill Creek itself (the same as Ballinger's), Taggart's on the Osage Fork, and Weathery's also on the Osage Fork. You will find references in Ramsay's online that will give you more clues to help you pinpoint the mill location that you want.

Lastly, the southern forces in Missouri massed in southwest Missouri were desperate for foodstuffs for their thousands of soldiers, and both armies had clashes at various mills, stores, and villages in Camden, Laclede, and other area sites in the fall of 1861 and the winter of 1861-1862.

Oh, I almost forgot about that "massacre of the Home Guards." You can forget about the Salem fight of December 3 in Dent County, since Mr. Bohanan died that very day in St. Louis, which would be impossible for Bohanan to attend. My guess would be that Bohanan took part in the "Battle of Cole Camp" on 20 June 1861 in which southerners from St. Clair and Cedar Counties led by Colonel Walter S. O'Kane surprised hundreds of mostly German-American Union home guards sleeping in two barns at three in the morning. Losses were heavy on both sides, but the home guards suffering casualties of 36 dead and 61 wounded were worse than O'Kane's about 15 KIA, 20 WIA, and 30 POW's. It's a stretch that some Laclede County men were in that Missouri State Guard force that day, but maybe it happened just that way. Unfortunately, some of the victorious MSG men slaughtered some of the German-American home guardsmen rather than take their surrender, giving rise to tha nickname for this gruesome action. I could be wrong, but that's my guess.
Bruce Nichols