"unengaged with me or the kids and will criticise and complain all day" - doesnt sound like a good reason to keep him in the house to be honest...
maybe jsut maybe - separating and ending up with situation where he has to eb one on one with the kids on set days might actually help him to take on that responsibility and engage more.
however it may be that separating wont change him any more than staying with him wont change him. unless he has a light bulb moment and wants to change.
proudnscary - i see what you saying - but the stepmother issues was maybe really just a continuation of your father being a bad parent and not looking out for you - it wasnt going to be good as it was and it didnt get any better, sadly. but the divorce wasnt responsible for the ways of the stepmother.
but sometimes, a "strong" parent moving on and out of a bad relationship can work best for the children.
op has a lot to think about - but it would be wrong to maintain the status quo because he might go off and get a really bad stepmother for the children...
dd loves her dad - of course she does and separating wont mean she wont be allowed to love him any more - he will still be her fatehr....in fact there is a chance she may get more quality time with him if he steps up to the mark...
if he doesnt step up - well no differnet really to now, but op will eb stronger and happier and a house without moans and complaints and criticisms is far far happier all round. getting away from that - is like a cloud lifts. my exP hasnt changed since separation - if anything has got worse - he may blame the separation for that (he cannot cope with it...) ... i dont. i think he would have got even worse anyway staying in same house.