First, I agree that only the main character's origin story should work in SP. Every other orign story related content for other group members should be blocked in SP and become invisible.
I agree with this. I understand why some people like to have all that content available so they only have to play it once, but I don't agree. Picking an origin character should be a special choice, you shouldn't get almost all the experience seeing the origins you didn't pick. That just makes it less special.
The rest of your idea sounds pretty good as well.
