Just want to echo everyone else Bree - you did nothing wrong. This is all down to him.
Re. the PILs - yes, they probably are a little torn - especially if it looks like the cock wants to come back and you reject his advances (completely understandably, btw). But keep talking to them, let them know how bad you are feeling - if they know how much damage their arsehole son has done, they might pass it on to him, maybe some level of understanding might dawn on him. Of course, you can still be strong and brave for them if you prefer to do so - but they need to understand that he has done real damage.
Make sure you eat and sleep as well as you can - it's hard but it needs to be done.
Glad you've spoken to the school, it's another level of support for the children that they are likely to need.
Have you been to the doctor yet? I would suggest you go, just to let the doc give you a brief check over and so that you have another back up - you might need to consider some counselling because in the end, we and the dog can only do so much - you might need a RL person to sit and listen and help you find ways to get through this and your GP might be the person to point you in the direction of a counsellor.
Remember - none of this is your fault. It was his choice to break his marriage vows, his choice to walk out on his famly, his choice to try and kill his unborn baby, his choice to have an affair - he is the one in the wrong. And it seems that he's not that uncommon either - there is another thread going at the moment where a lady is 4m pg with their second child and her OH has just walked out with no warning, just packed his bags and gone. And a few other posters have come onto that thread saying the same thing happened to them. :(
I don't know what goes through their heads when they do this shit but it is THE most selfish thing imaginable.
(((hugs))) for you my lovely - cry when you need to, let it out, and then deep breaths and keep going. You can do it, you know you can.