When do I start talking to him about the affair and potential other affairs? When do I start figuring out if reconciliation is possible or not? I'm in a limbo with him where we don't talk, don't work through anything that he did and that happened and I know that he thinks that will eventually lead to me 'getting over' it and us just pretending nothing happened..
When do you start talking about the affair? You don't. I assume you've confronted and that's why he's moved out. He knows you know, and he needs to become remorseful. If not, and you R, then effectively he got what he wanted - he waited you out, and then you took him back, basically pretending nothing happened.
This isn't what you want to hear, and it's scary as hell, I know, but you need to let him come to you with ANY talk of reconciliation. If you bring it up, you're saying (without actually saying) "I know you cheated, but I still want you back, so cheating is ok as long as you don't leave." I know that's not the message you want to send, and it's not the life you want, so please, for you, don't do it. Don't discuss anything other than the baby UNLESS it is him apologizing. Blaming you is completely unacceptable. Waiting for you to just 'accept' it and ask him to come home is unacceptable.
So what happens if he never does that? Well, you have your answer. That's the scary one, right? What if this is it? What if he never comes around and apologizes? None of us wanted to face that possibility, but if you want to move forward, and not just rugsweep, then you need him to be the one to initiate any talk of R.
I suppose the one thing you might do is, and this is only one time so he knows this is where you stand: You could text, once, saying something along the lines of
"I have known for X days that you have had an affair. In that time, you have blamed me, blamed baby weight, insulted me and not even had the decency to apologize. If you are not willing to take even the smallest steps towards healing the damage your affair has caused, I am going to start planning my life without you. I will not wait for you forever, and I absolutely will not act as if your affair has not done significant damage to me and to this marriage. If you want to discuss this, I am willing to have a conversation regarding my needs, boundaries and requirements. I am not willing to be insulted, belittled, or blamed. You chose to have this affair. I will not be blamed for your choices."
I wouldn't offer him any more than that. Let him know that you're serious, that waiting it out won't work, and that you're prepared, if necessary, to move on. He may respond with something nasty at first. If he does, ignore it. He'll probably stew on it for a day or two before anything real happens.
Forgive me - but if he's waiting for you to just 'move on', I'm guessing that in the past that's how the fights have gone - you get upset, he becomes abusive, you eventually cave, and you both just move on, business as usual. Is that correct?