The Extra Media Map is there on the game page under Extra Media (See page
here. Multiple extra media images can be added (as far as I am aware of no real limit), hence the script always asking for another image. To edit the link, you have to use the edit game page script. Circuitous, but unfortunately that is the way it is... for now.
For question 3, there can be only one manual image. Other manuals need to be submitted for now, as extra media.