I will give you a sneak preview in order to attempt to account for the August 24 death date on Zimmerman's tombstone. I have sharp actions August 23 and 24 in 1) Jefferson County, 2) Washington and St. Francois Counties, and 3) New Madrid and Pemiscot Counties all involving southern guerrillas of this region. My best guess would be that George Zimmerman MAY have been mortally wounded in an action near Grosby evidently in Pemiscot County on 24 August.
Here are what details I have. According to the Kansas City daily newspaper September 2, a guerrilla band raided the northern settlement at Point Pleasant, southeast New Madrid County, on August 23. The raiders then rode south and camped near Grosby somewhere in Pemiscot County. They didn't know that some of the irate northern sympathizers from Point Pleasant tracked them to their campsite and attacked them August 24 killing three guerrillas outright, mortally wounding one, and recovering stolen property, horses and mules, 5 navy revolvers and carbines. The Point Pleasant civilians suffered no casualties themselves according to the newspaper article. I wonder if one of the dead guerrillas was Zimmerman. I eliminated the actions August 23 further north since they either involved guerrillas from those areas or no guerrillas suffered harm in those actions. This action seemed to be the best bet. If Zimmerman was killed, perhaps someone carted him home or his family was notified and went to retrieve the body. This is conjecture, of course.