Magical Secrets
Spells from lists other than the Bard spell list can only be replaced at one of the levels where the Bard gets Magical Secrets spells.
Spells from the Bard spell list can be replaced every level, even if the spell was gained at a Magical Secrets level.
http://community.wizards.com/forum/rules-questions/threads/4187916#comment-51846041
Dark Delirium
What's the point of the charm or fear? For the duration, the creature can't see or hear anything except you, itself, and the illusory realm. Who cares if its charmed or frightened if it can't affect the material world? The charm and fear is superfluous. What's important is the 1 minute mezz.
The Great Old One Patron
Note that one suggested source for mind flayers and intellect devourers is the realm where Great Old Ones live. Yes, mind flayers are creatures of the Cthulhu Mythos and now so is your warlock… Ia ia shub niggurath.
Create Thrall
The feature doesn't say you can remove the charm yourself but I'm ruling you can, as a bonus action.
School of Enchantment
Hypnotic Gaze applies the stunned condition.