Although he is the baby's father, labour and birthing are very physical things happening to your body and it may involve medical proceedures or you taking strong pain relief which could make you feel vulnerable. It certainly can involve at least partial nudity. I say this not to disempower you in terms of your birth choices but to empower you to see it as something private which is happening to you, he has no right to be there. Indeed if he would make you feel stressed and uncomfortable, things are likely to go more smoothly without him there.
I am happily married to DH and had a very quick, straightforward birth with no pain relief, DH was with me the whole time. When DS was born, I held him and gave him skin to skin contact for the first 50 mins of his life, whilst I was being stitched up from a 2nd degree tear, then DH took some photos of DS and I before he got his first cuddle. He was really respectful of what DS and I needed after the birth - calming skin to skin contact, the opportunity to have a first try at feeding.
If DH had been an ex, I wouldn't have wanted him there. I'd have felt pressured and uncomfortable. Definately don't feel you have to make a decision about this now and you can change your mind if you have already said it would be okay and don't feel you can be honest with him and have him respect your decision. Just don't tell him when you go into labour, say it all happened so quickly (even if it didn't) and phone him once you are ready for visitors.
50:50 care of a newborn baby is not practical, and pretty impossible if you are breastfeeding. I'm sure there will be advice available on what is considered reasonable in the early days. Given that he doesn't sound that interested in the baby now, he may have lost interest in six months time. This wouldn't be your fault (even if he blames you), if he wants to be a good dad then he'll have ample opportunity to be one, if he chooses not to be, nothing you do can make him one.
He sounds like an emotional abusive, cheating git, well done for chucking him out. Concentrate on you and your baby. I'm in Scotland and am unlikely to be close enough to take you out for a coffee, but if you are then feel free to send me a message and we can see. :)