1,3,4: Burrow range is calculated from either plague worm initial space to any plague worm final space. So in our 1x8 hallway:

P= plague worm

O= figure occupied spaces

E= empty spaces

EPPEEOOE

The worm can burrow and move the figures such that

EEEEOPPO

"Each" figure because more than one figure can be moved, one per space.

2: those npcs can only be moved if the special rules say they're figures- the plague worm can only burrow into figures or empty spaces.

5: if the marshal is going to cancel dark charm, she must do it before the will test (for dark charm) is made, as at that point the OL is already resolving the card. By the book must be used when the card is played.

But yes- I would say the OL could play word of misery during the effect of dark charm. For example, if he were forcing a hero to perform a move action, he could play word of misery before marching that hero throuh lava.

During a Dark Charm, the hero is considered "as one of the OL's monsters", so would Word of Misery still apply?

He's not a hero during that action, and the damage from Lava would be taken during his movement, during that action.

You're right, bad example of a reason to play Word of Misery. It remains a valid time to play the card, however.