Durin's Bane "cannot leave the staging area" (by its own text).
I presume this means that O Elbereth Gilthoniel will not work on Durin's Bane (since putting DB on the bottom of the encounter deck would involve it "leaving the staging area"). This seems fair enough.
However, the same reasoning suggests that Dark Pit will not work on Durin's Bane, as it "discards DB from play" (= "leaving the staging area). This would obviously be absurd as DP appears to be the way that the quest requires you to overcome Durin's Bane.
Given this, what reason is there to say that OEG does not work on DB, but that Dark Pit does? (other than perceived designer intent, a very poor basis for rulings).