FrogReaver
The most respectful and polite poster ever
If they're going to find the letter no matter what they do, their agency becomes no more than an illusion. Same is true if they're going to meet Brash the Ogre no matter whether they go left to room 6, ahead to room 11, or right to room 14.
Meaningless choice (ie agency) is an illusion as well.
Pick left, right or neither is the choice. You have full and total agency in making that choice. However, it's a totally meaningless choice/agency because you have no information by which to make a meaningful decision.
In other words, taking away meaningless agency isn't something to be frowned upon. Do that as much as you desire. However, never take away meaningful choice as that's the path to railroading and the actual illusion of choice.