Gabriel- Yes our MIL problems are almost identical. I had not talked to mine for 8 months when I caved and went down there. But it did me no good so OH WELL
Like I said before though, just always be positive about MIL when talking to W. Even if W is complaining about her, just listen. That is how it works for me with H. Then they are more apt to talk about the problems there.
As far as the custody arrangements, whatever is stated in the D....stick with it and make W stick with it as well. Don't be a doormat for her. I know you love S5 to death but you also have a life of your own now too.
W will probably want you having S5 more than her which will keep you from GAL of your own and therefore she will know exactly what you are doing.