[Soothsayer]

The only thing I can remember right now that might help....although youll have to do some maths to apply it to your situation:

I completed IWD (no addons) on normal difficulty with a FOUR man party, one of whom was a multiclassed mage, and she definately did get 9th level spells, she had them for most of lower dorns i think, perhaps even earlier.

I know this isnt exactly what your asking for, but it might give you some idea.