Okay, first things first . . . this is a mess.
A lot of updates, so I'll just jump on this one to give my thoughts.
Reality #1: A termination would obviously be the most convenient solution here. You also really obviously don't want one. Posters can suggest it all they like, but it's your choice at the end of the day and it's clear where your heart lies. So. If all goes well, you're having this baby.
Which means it's time for the hard choices.
Reality #2: Your ex-DH may want you back now, but it's incredibly unlikely he still will, once he knows you're having another man's child. He didn't want a third child to begin with. He won't want to raise someone else's. So, good news is, your conflicted feelings towards him don't really matter anymore. You mostly wanted back with him because you felt sorry for him, and sorry for your 13 year old son. Not because you actually wanted to be with him. I don't think so, anyway. It seems like you're being driven by guilt, and actually prefer this other man.
Your marriage ended for a reason. Let it lie.
Reality #3: In this situation, you have to make a priority of the person who is the most vulnerable. And in a few months time, that's going to be this baby. Your children are teenagers. The child who lives with your baby's father is also a teenager. It's going to be brutal for all of them to have their lives upended this way. They will all resent you. They will all be hurt. But at the end of the day, they are all far closer to maturity than the baby you're about to bring into the world. That baby needs to feel safe and secure, and part of a family, from day one. They're going to be too young to understand all the upheaval and be reasoned with, the way the teenagers can, so they need more protection. That's just the way it is.
If you want the father in your baby's life, as they deserve, it's going to mean upheaval for all the other kids. It just is. Someone is going to have to move. It is horrendously unfair on your baby to make their own father a stranger, who suddenly moves in at age three. It is much harder for a child to adjust to all that instability than a teenager. Don't imagine this baby as a blank slate who's not going to absorb all this and be damaged by it. What about when they grow up, and find out dad missed their baby years because he put their 16 year old sister first, and their brother didn't want mum dating a new man? That would do a number on any kid, frankly. They will think neither of you valued them enough to risk upsetting their teenage siblings, and they won't be wrong.
If you're having this baby, then yes, you're going to be lobbing a grenade into everyone else's lives. But that doesn't mean your instinct, of trying to make the baby fit around them, to cause the least inconvenience, is fair. It's not. It's not fair on the baby, who is a person here too now, and will need to come first.
If your partner can't move, then you will need to move, or find a compromise between the places you both need to be. Your DS will have to finally accept dad isn't coming home. Your DD will have to struggle with a crying baby triggering her noise sensitivity ten times a day. Your ex will feel you led him on and be angry and resentful. Your new partner's relationship with his daughter will suffer if he has to uproot her. All of this is unpleasant, but it's the consequence of yours and DP's actions, and you'll both just have to toughen up and struggle through it, I'm afraid.
Good luck.