Thank you all for your comments.
I have spoken with DH and started the ball rolling on getting my state maternity allowance (although I worry it won't come through in time).
DH has agreed that all 3 children should get an equal share if the house and that, should anything happen to him, I would be able to live here until they come of age. I don't know how much I trust to hold him to this and would like to get this in writing, but for now, I feel better that he seems more reasonable about the house and will treat our newborn as equal with his DC.
I will also be getting life insurance on him to cover support for myself and baby should anything happen to him. In addition, he has agreed (at least in theory) to opening some sort of joint fund for the newborn's exoenses. I will believe it fully when it happens as I can't see this happening and I am interested to see what he thinks is a reasonable amount for day-to-day expenses but it is a step forward and already feels like a huge relief. Even if we are both expected to contribute equally, I will hopefully receive enough MA to cover my side without relying too heavily on what little savings I can manage to gather before baby arrives.
As for work, I have tried to make clear that I intend to have at least 6 weeks completely off (except perhaps for a weekly call with client to see how things are progressing). I have found someone who may be able to work 1 day for me, who I will have to oversee, plus a work experience girl who is a distant relation and keen to get into my sector who I am going to try and pass a couple of lightweight bits to. I have also arranged for 1 day from a graduate per week to cover admin whose pay will be covered by one of the client's sponsors.
In short, I will have to keep my hand in slightly but nowhere near the 4 days I feared. All this means I will hopefully be able to ask for some pay from client during my time off and, if I can keep below 10 full days over 6 weeks to maybe even 10 weeks (which would be much much better than feared), I should still be eligible for MA.
I haven't yet broken the news to husband that I won't be getting my normal income and I expect he will think I'm mad but I don't care.
I haven't told my family about our situation as I am hoping that by standing my ground, we can get through this. This is already a huge breakthrough for me mentally.
I won't be investing in my own property but as soon as I am working again I will be building my FOF (fuck off fund - it's a TED Talk thing) so I have at least enough for a deposit, a few month's rent and basic spending if I ever need to get out fast.
I have also made clear to DH that I don't want him to reduce his hours to help with childcare until baby is a few month's old. I may not have gone back to 4 days by then, if I can arrange for the support I've sorted to continue for longer. If nothing else, thus removes my dear of him being able to claim that he's a primary carer. He hasn't agreed to this and it is difficult to prove how many hours he works/has taken off for childcare, but I have now got access to his diary and, I believe, all bank accounts so would have some evidence if push came to shove.
Re. bedrooms, we do have a 3 bed house but one bedroom is for his DC and one is his home office. Our room is big enough to split when needed (although dreading this) but I am working on convincing him to adapt his do it can be a child's bedroom in the hols (only time when his DC visit and he doesn't work) and then revert to office for the rest of the time.
May have to split our room anyway eventually as the stepkids are one bit and bone girl, so won't want to continue sharing for much longer.
Feeling better. Not yet sorted and MY trust in DH after recent arguments is seriously depleted do will be trying to maintain at least some financial independence and full mat leave is maybe a luxury that will cause more stress than it is worth.
Time will tell. I am going to get dishwasher plumbed and damp fixed independently. It should be a shared cost (& damp should be his cost to cover given my lack of a stake in the house) but I just want as few stressors as possible for when the baby comes.
I know we still have serious issues but I also know that I am strong enough to either fight our way through this and reach a fair enough compromise or to leave. I would have liked all the above decisions to come naturally and instinctively to DH but I have married him now and he is the father of this baby and I do still love him. He hasn't yet broken that. Though he has come close. The truth will out when the baby comes and if he lets me down, I will be going to my parents. As fit and active pensioners who love kids and with a couple of spare rooms, they are at least a safety net which I am very lucky to have. I just want to avoid it getting to that unless absolutely necessary.
I will update again if things change for the better or for worse.
Thank you again - your fury and accounts of how difficult little ones are has opened my eyes and made me strong enough to get a back up plan in place and face difficult conversations.
Childcare after first couple of months is still an issue but I am working on that.