Aye, that gave me quite the headache to implement properly...
For techies: https://gitlab.com/riovir/divinity-2-planner/blob/master/src/store/build/build.js#L33-47

I think all talents work now that have a visible impact on the stats you can see.
If (de)activating a character is needed, I soon follow up with some sort of a toggleable campfire icon on them so you can see how your stats look like when they are not in the party. I just wanted to wait with that until I can add some avatar images.