I've been playing with my partner and I think I have experienced all of this. One thing about the way Larian handles couch co-op with split screen is that the "first player" is the main character. If another controller isn't connected you ended up playing the other person's character just like you would your own. That means that you won't have a specific backstory for that character like you would for one of the other characters like Astarion, Gale, etc. But you can switch between solo and multiplayer as much as you want. In that regard it feels pretty seamless.
One thing I've never tried is seeing if I can leave her character at camp. I have a feeling you can't and you have to use that character.
If she does want to join mid campaign and she hasn't created a character then I believe that will be an option. My understanding is that anyone who joins your campaign at any time has to create a new character.
Hope this helps!
