I assumed the Overlord monsters behind doors didn't activate until in the heroes' line of sight; yet I don't see how the conditions can be fulfilled for the OL unless you bring up all monsters at setup.
So do the heroes get the full info of objectives and maps and placements at beginning of scenario?
You assumed wrong. A change in 2nd edition is both the players and OL knows everything in the quest book unless otherwise noted. But when there is 'secret information' I find neither the OL nor the players get the know. =)
So the goblins can start escorting the prisoners into the torture chamber right on turn 1!