Structural Class Changes (Mana, Talents, Clear gish structure)

[Ugganaut]

Well, the Theurgist Wizard exists in 3rd Edition and was even considered in an UA for 5e. It's basically a Wizard that mixes their arcane study with study of the divine.

Never saw a Theurgist in action, or most likely our DM at the time didn't allow it. There was only one wizard spell I remember doing healing, and it was very costly for a small gain compared to cleric healing. I like that distinction personally. Having said that, I wouldn't mind if Theurgist was a school. Instead of getting better with your speciality school spells, you'd gain access to the Life theme - that would be the benefit.

AoEs scale much slower than single target as their area also scales in addition to their damage.

Makes sense. Thanks for the clarification.

Marcloure has a good response for this above: Arcane casters have been doing Divine casting for a very long time. :)
Note: Your options make Grimoire and even Spellbook nearly useless.

I'll have to have a good read of them when I get more time. From memory, Grimoire lets you get spells from any theme, not just ones you know. Even if all casters had 20 themes at level one, there are still 13 they don't, so it's far from useless. Using my restrictions to a handful of themes, this would be an example of something that bypasses the prereqs, making it very useful.

The short cut is the patron, but that's not related to their casting stat. Charisma does not fit the flavor - read the RAW flavor text of the Warlock. Pathfinder's Witch also uses Intelligence.

Read the RAW text. It says seeker of knowledge, but that's not necessarily the same as an academic/scholar(Mage). Studying and understanding books is Intelligence. Dealing with patrons and striking deals sounds like charisma. "Drawing on the ancient knowledge of beings...", they aren't learning it, it is being bestowed to them. Like knowledge being implanted in your brain. The learning and understanding has been done for you, and they get unique patron bestowed abilities that mages don't have access to.

Edit: "Occultist and Mage have similar flavor in their thirst for knowledge, but different flavor in the source of their powers. I'm inclined to combine them somehow, but not sure it'd be possible to do."
Not just the source of their power, but also how they attained it as I mentioned above. I hope you don't combine them, they are very different imo.