Hi Preggo, I've been in very similar circumstances so I'm just going to tell you what helped me get through being pregnant and on the verge of D.
1) I didn't worry about getting everything sorted out by my due date. Your baby and your 1 year old wont remember any of what happens for at least the next year or two. If possible set your sights on a much further out date than Oct 30. to have your marriage back on track.
2) I pretended OW didn't exist and did whatever I'd normally do if she didn't exist. I let H do his thing and took comfort knowing that their relationship would self destruct eventually. Try not to focus on her or on getting her out of the picture. Just be your best self and focus on what you can do to draw him back to you.
3) I smiled smiled smiled! Hormones make it tough when you are preggo... But trust me, you get the upper hand if you can be positive and upbeat at all costs right now. I was a duck... Everything rolled right off my back (even if I was dying inside ). He wants to separate? Ok, go for it. He wants to do his own thing some days... "Have fun hunny. We'll see you when you get back." Your H is a mess right now but you don't have to be.
Lastly, I had my first son alone while my H was deployed overseas. He came back a very different person. The day I told him I was pregnant w/ our second he told me he had gotten someone else pregnant as well. He made some really bad choices during this time. However, now looking back I see that it took him a long time to get back to being the guy he once was. I don't want to give your H any excuses for his bad choices, but in hindsight I wish I had been more supportive and had given him more time to sort through his own crap between deployments. If you can be strong for him now it will pay off later.
Is it fair that you, the pregnant one, has to put his needs first right now by giving him space (not pressuring him about the baby or OW)? Heck no! But if you can be strong and give unconditional love even when it hurts you will have a much better shot at getting your H back.
Hang in there!
BD: Aug 2012 Separated since May 2013 S born Aug 2013 Aug 2013 H agrees to consider 'baby steps toward working things out' H is/was actively seeing someone?