It cannot be accessed at any other point in the game.
Requirements for Access Edit
- You must have Caedmon in your party.
- You must have completed Part One and Part Two of The Silver Falcons quest series.
- You must have passed through Fornstrand during the main story.
If at any stage all enemy units are defeated, following waves will be bumped up the turn order, so you will always have something to fight.
These are almost impossible to calculate specifically. You should end up with +20,000g and some of the Vultures in the Assault Squads in wave 2 will drop Avian Fiend Feathers.