Does your dad know you posted his letter on a massive Internet forum to be judged? You've already made yourself incredibly identifiable and it's a huge breach of privacy, whatever the situation. I would get that taken down.
FWIW I don't think this thread is going to do you much good as this is something you are going to have work out yourself and deal with it, whatever your decision. There is always support and advice on MN but here there are so many different views of the situation and no one is "right" as such. Independent counselling in RL may be more helpful?
I think this immediate crisis has highlighted the huge problem you have in your life, which is a lack of independence, which is also behind all this trouble with your parents. I remember your story as I have gone through something a little similar, so I am not unsympathetic at all. I know what it's like to lose everything professionally that you have worked for, and your security and way of life, and the way you see yourself. The guilt especially that your DCs lives, as well as yours, are different to what you had planned.
Again, this is something you need to get support with in RL, it's hard to face up to the huge adjustments in your life. But the fact is you are living in a four bedroomed house, with two cars, one of which is a BMW, and your parents are buying your children food as you can't afford to. There are some huge changes you need to make here whatever you decide about the pregnancy.
You don't seem to be addressing the issues of your finances at all, as the pregnancy worries are (naturally) consuming you. But if the finance issues had been/are sorted you may not be in such a bad position where your parents are threatening to withdraw support you have to rely on. I suspect like me, the finances are a huge elephant for you as they relate so much to the changes in your life and your DCs life, and perhaps you carry a lot of guilt (which you shouldn't).
There is a lot of speculation here as to whether your DH is financially abusive or you are both just burying your heads in the sand, and no one here can tell. I do think if you face up to the financial facts and make changes that you have to, you will actually feel a lot more happy and settled when you come out of the other side.
I know it's shit and it's unfair and sad, but your current situation is not sustainable. The fact is that the stress your parents are causing by threatening to withdraw their support, is also of your own making - with a relatively well earning DH and two DCs you have to take responsibility for their basic needs and not rely on others. Yes your circumstances have changed through no fault of your own, but it is what it is, you are a few years on and you and your DH are adults with children and have to take responsibility for your lives. There are changes you will have to make whatever you decide and I think proper independent support in RL would be really beneficial.