Firstly I think you are incredible, and far kinder a soul than I am. You are a Wonder Woman already.
Secondly, I think your partner needs a vasectomy yesterday.
Third, lose your shit on a daily basis to all the agencies that can possible help you. They are taking advantage of your kindness, and what do you do - given how kindhearted you are when faced with the possible request to take on the next baby your partners ex is pregnant with? I think you'll guilt yourself into a yes, even though you say no way now. Stand your ground and say help me, or find other arrangements for these children.
I do not think that an abortion is the answer when you are already having severe MH issues - that option has a strong potential to break you completely. Especially considering how you feel already. Then your kindness at the expense of yourself and your children means nothing because you have completely destroyed yourself.
Now comes the bit that will get me flamed, and yes I am coldhearted and a bitch.
I think you desperately need to stop thinking about everyone else for a bit and put yourself first.
I think you should put the oldest into alternative care. Where is the fathers family in this? Not the father but his relatives?
I think if you abort your own child for this one and the other, you will both never forgive yourself , your partner and come to resent these children immensely . And what if they get returned to their mother next year and you've had an abortion based on a maybe?. Unlike a previous poster, I would not sacrifice my own child for the other two. Sorry, just would not especially if the concept of an abortion was throwing me for such a loop and I was unstable mentally already.
But alternatively, if you choose to keep your baby ( which I would ) and the others...some suggestions .
Your husband needs to step up, change his hours and pull his head out of his arse. Passively worrying with out actively doing anything is useless. If he is shit with money - you take it over and give him a sum each week. He needs to be engaging with SS as well as you. They are his kids.He needs to be all over this, he has breaks at work - he can get on the phone.
He needs to get home of a night and take over and clean, and let you out for a walk , a sleep or any kind of a break. End of story.
You may need to learn to drive given your isolation - given that many small kids there are going to be occasions you have to - accidents at home, illness - talking them to the doctor etc.
And if you cannot put this bring yourself to put this child into care, can you get an au pair? Or a local teenager to help a couple of times a week? Anything.
Flowers, love and awe.