I am not an attorney but was told by someone who is that if the custody arrangement you've had during a separation is working well for kids that it's less likely to be changed by a judge. And there is no way on earth I'd agree to week on/week off to someone who cheated on and divorced me, I didn't become a mother only to have my job cut to part-time without my agreement. So if you don't want that schedule, don't agree to it. I don't see how you could do worse than that if you went to court.
Your sitch reminds me a lot of my own, except OW lives a few hours away. My H and I are separated, I have dropped the rope and he's been awfully friendly lately. Am pretending not to notice since he's still involved with OW as far as I know. He also thinks our S will be happy that he's happy - except WH is not and has said he's not a few times. Doesn't look happy either, he often looks depressed and he's gained weight, etc. So I have to concur with all the setting them free advice, and turning the whole thing around on them by making your life really good - GAL, 180s, and looking your best. Somehow I've got the happy life WH thought he would have, but I had a hard time getting there while still living with him, I'm sure that's really hard.
M: 35 H: 34 S: 8 Married: 12 years Together: 16 years Bomb: 11/2/09 Sep: 1/1/10 EA confirmed: 11/2/09 PA confirmed: 3/28/10