I feel sorry for the Mum and the child in all this, 6 months is not long and his ex and child will still be getting used to the big big change in their lives, having your dad leave to shack up with someone else is very confusing to child and there will be a lot of anger still surrounding why he left, you and your partner may well be all happy and loved up but I bet it will take a lot longer for his Daughter and ex to be in the same position. They are still adjusting.
If you are questioning finances in regard to having a baby now I'm assuming you're planning this to happen sooner rather than later which I think is a big mistake, why cant you wait a few years and let the dust settle, let everyone get used to their new lives and become settled.
If he was so unhappy for "years" he could have left a long time ago, was his wife aware of how unhappy he was and how much trouble her marriage was in? or do you only know your partners version of events. I would not be suprised to find his wife knew nothing it and to discover he was still sleeping with her.
Is the access a problem because she doesn't want her child at your home or around you? I can understand that a little. Can your husband not see his Daughter alone each week either in the family home or by taking her out somewhere, just until everything settles, the amount he pays should be changed because of the amount he sees his child, they are not linked.
He has agreed %30 of his income whilst knowing he was setting up a new life with you so he must have done some sums in his head and decided this was acceptable, it will cause a shit storm if he reduces it now, especially if he says its because you both want another child as its still so raw.
I agree that when a together family have more children there is less money to go around so the amount spent on each child lowers but its not as black and white as to say the same should happen in seperated families, two seperate amounts of rent need to be paid, two amounts of gas/electric need to be paid/ two amounts of clothing needs to be bought etc wheres in a together family only one lot is needed.
I just think its a bit of a mess for his ex and child and would be rubbing salt into the wounds if you cut money so you can have a child together, Id wait a while and re discuss in a year or so. Im not a bitter ex, an other woman, a lone parent etc, its just the way I see it as an outsider. I really really hope you dont end up posting in a few years being in the exs place as he has left for someone else, that fear would always be in my mind and I dont think I could have children with him.