My travel agent got a competent Delta representative on the phone and posed my same questions. The representative's response, roughly, was "if it ain't Delta metal, you ain't upgrading".

So I conclude my options are;
1. book in Y or S class with Airfrance and use my miles to upgrade on Airfrance.
2. book an M class ticket to Europe on Delta and use my miles to upgrade on the Delta flights, and then travel in economy on AF to West Africa
3. book an I class Business ticket to Europe and a regular economy ticket on AF to West Africa.

Thanks for the feedback from everyone who posted.