Se7en/WMG

The film takes place in Gotham City.
It's a geographically indeterminate urban sprawl which vaguely resembles New York and/or Chicago, dark, gothic & decaying, in which it seems to be constantly night and/or raining, not to mention its populace seems about as damned as that of Gotham.
 * Yes, of course! The events in the movie took place before Batman was born (or was still in the process of completing his training). And Somerset did stick around like he said at the end of the movie; he just changed his name to Lucius Fox.
 * And one day, defying all current medical knowledge, Sloth walked quietly out of the central hospital with a smile on his face. He carved it there himself.

John Doe is actually and Author Avatar
Think about the various speeches he gives about his work. How it will be studied and followed forever. Fincher's basically goating to the audience through John Doe, telling us that his work (i.e. the movie) will be remembered forever. And I can't disagree with him.