@waffletots Firstly, I'd like to offer you my deepest sympathies for having to learn your husband has been having an affair when you're 38 weeks pregnant and Christmas is just two weeks away. You sound amazingly strong given this bombshell that's happened, and I admire you for that — many people would fall apart unable to even think — let alone act — and become either hysterical, flare up out of control, or simply curl up in a ball barely able to function. For you to pre-think all your moves, even down to ordering STD tests online, arranging for your three-year-old to stay with your parent's, call round your brother and SIL, pack your husband a bag, plan to send the bombshell texts to him when he pulled up on the drive — I can't tell you how amazing you are.
And then to arrange an appointment with a solicitor within just two days is nothing short of incredible. None of us know how we'd react to such devastating news, but I suspect your husband will have guessed your reaction having been together five years. You sound the calm and collective type and that's a good thing, especially whilst going through this at such a crucial and emotional time.
One thing I noticed in your first post was how you said your husband had occasionally mentioned this OW and for the benefit of some who may not know, this is usually always a big red flag that the woman is on his mind and he has a crush on her: why else would he be thinking of her and worse, mentioning her to you?
What he's done is despicable and no way is it forgivable — certainly when the affair has lasted one whole year. That aside, just because it has lasted a year it doesn't mean he's in-love with her, it's possible she was an ego boost to him for whatever reason and he just enjoyed the sex. Yes, that is absolutely wrong and disgraceful, and in some ways had he truly fell in-love with her against his control, although it's still unforgivable you could half understand why he did something so selfish and reckless.
It sounds, though, that he was just stringing her along, hence him lying to her and not telling her you were planning more babies, so I suspect when she discovered the truth she wanted to hurt him back — and so she did the worst thing possible by telling you. I'm not excusing her as it takes two to Tango, but your husband sounds cold and callous by how you've described his reactions, so you don't know what lies and promises he fed her and I'm sure she went into a tailspin too.
I doubt very much he's staying with her now as he must be furious with her, and he's been caught out by both of you, so at least you have the satisfaction of knowing he's effectively homeless until everything gets sorted.
It's brilliant that you have your family supporting you, but legally you're going to have to sort out visitation rights for your husband to see his children. I know it's hard, but that's the law. And I'm sure your eldest is starting to wonder where he is. So you need to arrange something quickly. I can understand you not wanting to speak to him or even see him, but realistically you're going to have to eventually. You can't not ever see him again — he's going to be around for the next 18 years and there'll be times when you'll simply have to communicate or see each other.
As for the marital home, courts usually always give the mother custody and always make sure they keep to stay in the marital home until the children reach university age. So you've no need to worry on that respect.
Finally, it may help you to seek counselling later down the line as this has been a traumatic time for you and you could unwittingly be burying your pain inside, which needs to be released at some stage.
Good luck for the future and I wish you all the very best.