I'm having a devil of a time trying to figure out the last encounter.
The BBEG has a spell list entirely built around convincing crowds to do her bidding. In Tier 1-2 and 3-4, how have you spent her spells to foment discord between the PCs?
I'm having a real hard time thinking of ways to use her 2nd level spells without getting immediately attacked, especially since her first action (likely Charm Person) is when she's invisible.
So, what spells does she even want to use aside from Charm Person?
Enthrall forces everyone to save but only delays the failed saving targets actions by d3 rounds. Hypnotic Pattern does essentially the same as Enthrall. Suggestion can remove people from the encounter if worded correctly, but Charm Person works just as well if not better.
Cause Fear works well against (Half-)Elves, Lesser Confusion's not worth casting, and Sleep will be situational.