I envy anyone who can play the autoharp! I have one in the house, but have never got to grips with it: my son picked it up and was instantly playing something vaguely musical!
As for playing and singing at the same time, it does take several parts of the brain all working at once along different pathways to do that. I couldn't do it when I first started guitar, but now I can: couldn't do it when I first started buttonbox, but now I can at least for some songs. I'd liken it to driving a car: e.g.when you are learning it's very hard to carry on a conversation with anyone, but eventually it becomes semi-automatic: you don't any more think about changing gear, although hopefully still keeping your eye on the road and aware of any traffic hazards..