Hello. If I were you I would read to see what the Union folks said about these places and the battles they fought. The unit histories
should shed some light on those actions. All those books have to be researched anyway, so perhaps you could do as well as anyone just by researching and write your own books.
There is a site in Illinois that is just outstanding with photos of the men and everything, so I would say check there. I know that quite a few men from Illinois were involved in fighting at Arkansas Post. My first cousin is descended from a man who was with the 123rd Illinois. That man's brother was killed there and the Union soldier later returned to live out his life in that area of Arkansas after the war ended. That one Yankee soldier left a ton of descendants in that part of the state. The irony is no one knew he was a Yankee until recently.
My ancestor's brother-in-law who was with the 36th Arkansas CSA was captured at Helena and never came back even though he was listed as still alive when the war ended. It has been explained to me that many men were alive but were not able to return home due to their state of health.
Good luck!
Linda