Niles,
You mentioned that your sources were mostly from oral history & the State Achives listing. I wonder if you've tried to order his pension application? If you order the complete file, there are often affidavits of witnesses, etc. who may have seen him get wounded. You can order the file from a link at the top of this page. They will be much quicker than ordering it yourself from the National Archives.

The Illinois soldiers that I've researched who were transferred to the Invalid Corps were sometimes stationed at hospitals (either field or permanent ones) and assisted there being unable to do regular duty. Others did "office work", etc.

With him being severely wounded, it seems likely that he may have been sent to Memphis, as Sherman "dumped" all of those who were unfit for duty there before proceeding onto Atlanta. He could have been stationed at Cairo too as there was a Headquarters there; and a hospital at nearby Mound City. His complete service record or pension application will have the details.

I will have to disagree with you on a couple of items on your website. N.B.Forrest was an early member of the KKK, but he was not the organizer. This has been proven. The second thing is that Grant wasn't the first to say "Unconditional Surrender". It was one of his Generals--maybe Thomas??--with Grant repeating in a telegraph what was said. The name of course was "tagged" to Grant because of his initials. I thought he explained this in his Memoirs, but it's been a long time since I've read them.
Ann