I understand that dh can't look after your dc three days of the week as he has to collect his own dc and can't be in two places at once but why can't he look after your children on the other two days?
I don't think your office should be given up, especially if it's needed to earn the living that's covering most of the family costs. I can also see how combining one of your ds rooms into an office bedroom type thing would be a problem if you need storage for important paperwork, especially if it might be of a confidential nature and wouldn't be practical, what happens during school holidays and the ds can't access his bedroom (they have much need for privacy imo too) because mum is in there working, this sounds impractical.
I do think the girl needs privacy, I say this as someone who had to share one room with my brother and step brother when at my dads, I hated it, esp when periods started and I couldn't get up to the toilet on morning and had to wait for them to fuck off as they'd see that I'd leaked etc. dad only had two rooms and it was only once a month we stayed but I think if he'd had a six bedroomed home I'd feel resentful, be that wrong or right. I really do think your dsd will need her own space at some point, she may be happy sharing now but that could change when puberty hits. I agree though that your dh should have also considered this before he met you but it is what it is now.
I understand people saying the dcs will feel unequal/unwanted but it works both ways, your dc could feel unwanted as someone is being paid look after them when they have a stepdad at home on two days of the week. I think you both need to stop seeing it as your dc my dc and think of it as our dc and you both need to make changes and compromises to make it work, not just you.
I think you either need to remove the need for live in childcare, if this is totally not possible then converting one room into two, or maybe the attic into another room, but that costs money and would take time, and it'll prob fall to you cover the costs because of you having to keep assets separate.
The bedroom thing aside, you also need to have a discussion with dh about house rules, bedtimes, chores, punishments etc and what is expected of all the children, and then it's probably a good idea to get together with all the children before moving in so you avoid arguments over on set of dc being allowed to do things the other can't.
It needs sorting though before they move in.