Thanks for giving me some possibilities here, Steven. I have managed to rule out most of these fellas, however.
George E. Buck probably continued to live in Gorham, New Hampshire, into 1900 at least; about 35 years after the George Buck in question moved to Wisconsin, signed up to fight and pretty promptly died what sounds like a painful death from pyaemia.
I haven't researched enough to confirm it, but George H. Buck may have lived until at least the 1930's when we find him living in California. The marriage record you found may be him or it may be a transcription error for the more likely George W. Buck. His middle initial appears as W on all other documents. Since familysearch doesn't tell us where the marriage record comes from (besides "Maine") it's difficult to confirm much from it. Both George Buck and Hannah Kelley were common enough names in the area at the time. The marriage record of George Wilmer is that of my great-great-grandfather, though. I definitely need to research this guy more.
The George Buck you found in the 1850 census appears to be George A. Buck, who seems to have continued living in Fairfield until at least 1880. This is about 15 years after the George Buck in question is supposed to have died in Nashville.
The last George Buck you found in the 1860 census probably died in or around Vassalboro in 1869, which explains why he dropped out of the census record. Again, this doesn't line up with the known facts of our George Buck's final few years of life.
I appreciate your efforts. I'm afraid the census doesn't have any answers for me, though. The most likely guy I found is one George Buck living in Wisconsin during the 1860 census. I can't track him past that point, so he may just as well have gone to Maine as a very young man, married an 18-year old, then brought her back to Wisconsin and immediately left for war. I think it's a long shot, but it's a possibility.