Bob, though not definitive by any means but contributes to your point I believe. Maps of the battle published at the time indicate that the navigable channel clearly was north of the island. At the time the channel was considered the "boundary" between Missouri and Tennessee. This would make the island in TN. Further support to this is the 1872 Missouri atlas shows the island as part of TN.

http://www.historicmapworks.com/Map/US/40847/Bollinger++Scott++Cape+Girardeau++New+Madrid++Stoddard+and+Mississippi+Counties/Missouri+State+Atlas+1873/Missouri/

http://www.rarenewspapers.com/view/597800?imagelist=1

Then there is this 1793 map that shows it in Kentucky but it also shows the border of TN and KY running south of where it actually is today but Kentucky Bend and the island are clearly shown on it. Look just north of Reelfoot "River".

http://www.historicmapworks.com/Map/US/1609029/

For what its worth... John R