Dating his ex wife
Your situation differs because of the marriage and children of course. Those two will always be connected and that will never change.Have you expressed any of these feelings to your boyfriend? You should be able come to some agreement that works for both of you.I agree, a civil relationship with your ex IS great for the kids, but there still needs to be some boundaries. It sound like their boundrie are a little different than your's. He needs to make a decision does he want his ex wife back and play house, does he want to be single and still be able cater to his ex wife with no strings attached to anyone or does he want to be with you without an ex wife intruding on your relationship.
With a larger circle of friends(or a new romance of her own), she would be less inclined to lean so much on your boyfriend and that would go a long way to washing that ex right out of your hair. Do you think he is worth putting up with these feeings?
I think a witchy ex-wife would be more difficult to take.
I'm afraid if you push the issue too much, you will come across as the bad guy or, more precisely, the shrewish girlfriend and then the whole situation could turn on you.
So, you really have to ask yourself if you can handle their situation.
It works for them, but does it work for you and are you willing to put out the effort to make it work for you?