I have no answer for the cerebral palsy thing. I don't know anyone who has that. Otherwise I would experiment for your benefit of course.
I always think it's weird to ask someone about dating an ex. If you know that he was crazy about her and she dumped him hard I would avoid her like the clap. But, especially under the circumstances you describe, I would never as for his permission. In big part knowing that I am going to hook up with her regardless of what his answer is. It's said by some that you're obligated to ask permission and that he's obligated to grant it. I just disagree.
I fucked a girl while she was married to a good friend of mine. I was fucking her when he met her. I kept fucking her. He knew she was fucking other guys. At one point they broke up and she actually moved into another friends house. He still kept fucking her while she was with our mutual friend. Of course so was I. Eventually she moved out of that guy's house because although he's my friend I'll say he's a doushebag to women. We both kept fucking her. I knew about him but he never knew about me.
Anyways, my point being that when you're there first you still maintain your rights to her.
