I don't know of an old map, but I think you may be able to do more with the Section/Township/Range numbers.
The old county lines were established (and frequently changed) when population in an area reached a certain number.However, The Township/Range numbers were based on measurements relative to meridans of longititude/latitude, not on popluation.There is a main north-south meridian just about in the middle of the state -- that is, west of Gallatin.So 'Range 8E' is a square 8 'ranges' east of that meridian.Township 10S is the square 10 'townships' south of the east-west meridian.Within each Township/Range square, the 'Sections' are numbered from one to 36, always in the same order (same position in the square). (There are some very good instructional materials at the Illinois land database site that can make this clearer than I can.)
Even thought the township names may have changed, the Township/Range/Section numbers stay the same, because they are based on longitude/latitude coordinates.This means you can locate the Township/Range/Section even on a modern map.(There are some very good online mappers/aerial photo sites that let you see the terrain -- one is www.maptech.com .)
If you spend some more time at the Illinois land sales database, you can find the T/R/S location *and* the specific location withing the Section (ie, NW 1/4 = Nowthwest quarter of the section).This will allow you to find the exact location of your family's land.(You can even sort the land database records by Township/Range and find out who their neighbors were.)
Also, in terms of payment for the land,if you search at the IL Land database on your ancestor's name and then 'click through' on the highlighted name, you will find more detail including payment.In addition, the IL land database is connected to a service that lets you see a map of the townships. (Fun!)
I don't know if you've looked at the book by Glen Miner on the Cemeteries of Gallatin County (at Rootsweb GAllatin site, under 'Cemeteries').This wonderful resource has much more than a listing of cemeteries.If you take the time to go through *both* books, you may find more detail on your ancestor.
The 1812 & 1816 maps you mentioned just don't work.Very annoying!
If you have access to copies of the Gallatin Court Records and the Shawneetown Land District Records that you refer to (other than what is found in Glen Miner's book), I would greatly appreciate a lookup on William McGee (McGhee) and Rachel McGee (McGhee) who were early land purchasers (McGhee Cemetery) and who both had early court cases.