Mack-
I don't have any information on the Thompson's of Russell Co; however, I do have information on some of my ancestors who lived in Illinois...
In 1850, my family was living in Messac co Illinois, they ended up going on to Missouri eventually. Why they went to Illinois from Tenn.-I have no idea. I'm sure there is a good chance they had relatives living there, William J (the head of the household/my ggg grandfather) was born in Virginia, so there's a chance he had a brother living in the area (this is an avenue I have not yet explored).
After the Civil War one of William J's sons was living in Illinois. He was moved from MO to Alton Illinois (Prison Camp). I have not found any hard proof of him living in Illinois (though I did find a Charles J Thompson living in Illinois who could possibly be him-I try not to put people in the tree unless I know they belong there).
I think that William J, went to Illinois to try and find his son, Charles J, and there is a William Thompson (who fits the age of my William Thompson) living in Marshall Co Illinois with Abram Allen-working on Abram's farm. That is all that I know of my Thompson's living in Illinois. It would be great if we were related through a Charles J. Thompson...but I know it's a big chasm to cover with the Thompson name and finding relatives.