So if characters' full abilities are available, and those happen to include influence/control over dreams/nightmares, to what extent can those be utilized? I mean, besides not being able to wake up, obviously.

EDIT: also, I should note, that Merlin's immortality itself seems connected with his power over dreams. Specifically, that his 'true' body is more incorporeal than physical, and even if an instance of himself in a dream is destroyed, he simply can reform and recover rather than it being a true death. Would this have any impact on how things work?