Mmm, I think D with her own space sounds good.

I really think that you'd have to discuss and agree on everything first, like ways you'll manage disputes and other things - discuss who is the boss for each thing.
That's why I think it'd be valuable to involve an outside person - when I lived/worked in a respite house for youth, a youth worker would come for a meeting every so often and discuss how things were going - so if we were at an impasse on an issue, we had a fair and objective person around.