I don't know if you can beat the game without beating Marathon.
(For that matter, I don't know if you can beat the game at all yet; I'm working on the Deku Tree now.) But you certainly will want to have gotten the mirror shield, since there are just as many mirrorobes in the Deku Tree as there were in Marathon. (The mirror shield is also mandatory for the final boss of the game.)
As for how to get the boss key in Marathon:
Did you ever figure out how to get to these stairs?

To get there, head up to the far top-left corner of F2. (Bring your fire boomerang to disarm two rooms' worth of red bubbles.) Go south to a treasure chest containing the map, then south again. You'll be on a seemingly useless ledge. But there's an invisible bridge there (can be revealed by the lens), and that invisible bridge will take you south to that staircase.
The staircase brings you to F3, where there are a handful of rooms. At that point you'll have to bang your head against the wall a bit to find a false wall that isn't in the middle of a room (and also isn't revealed by the lens). Kill all the darknuts in the room you get to via the false wall to reveal the boss key.
As for Statues, it seems to be mostly useless beyond, I believe, a piece of heart. The secret is that Statues is effectively the basement of the Deku Tree. If you fall through holes in the floor in the Deku Tree, you'll end up in one of the rooms that are shuttered off from if you enter Statues from the front. And then you have to exit Statues and go back through the jungle to the front of the Deku Tree again. No fun.
Edited by praxdye, 29 July 2012 - 09:54 PM.