I'm at the point after killing the Gatekeeper and choosing to enter either the Mania or Dementia gate. I chose Mania because I wanted the mid-level mania spell. However, no matter which gate I enter (after reloading) I only receive the mid-level demoralize spell. What gives?

Solved! I was reloading the wrong save (duh!).

Try exiting the game and reloading. Sometimes some things don't get reset if you just reload the same cell again.