I think he's thinking more of his parents. From what you said, they need you there to pay them rent in order to pay their bills and eat... He can see his mum panic-ing about what they are going to do if you move out and doesn't want to have to deal with that stress.
He also doesn't have to spend a summer holiday (and easter holiday and half terms...?) there, day in, day out, having to be hyper vigilant while the kids are playing in effectively a dangerous building site where the other adults around don't think of them or the consequences of their actions. It's one thing for a grandad to slip an extra couple of sweeties to their dgc or let them run riot when they only see them occasionally - but completely different when they are all living together for the foreseeable future.
You do have all the income from the other two properties - you wouldn't just have to rely on your dh's wage to get your mortgage. And you could cut back mortgage payments again if you needed to, even just in the short term to live safely - far more important than paying off the mortgage when you have little kids! Have you looked around in the area that property A is in - sounds like an ideal area for you - is there anything you could afford there? Even if it is a bit smaller than you'd want as your house but one that you could move to for now and then swap with property A later on so that the tenant has a bit longer in there if you feel guilty about moving her out too quickly?
If you do the figures for that option too - and then show that moving into property A is better. Thus you'll have discarded one option and it should make property A a more appealing option.
I would also find out why he thinks that he gets to be the one that makes the 'final decision' about not moving out. You've tried living with his parents, it's not working. Mirror his language and if he says that he is not prepared to consider moving out, then say that you are not prepared to consider staying with your PIL as an option. You have come up with a suggestion that he has shot down (living in property A). So make him come up with a suggestion of where you could live and how the finance would work. Make sure he leaves the fact that his parents need your money out of it - the fact you have been there this long has been a bonus. Would they have bought the house and expected you to come and live with them if you were happily settled and had your own house (is property A your house that you've moved out of in order to live with them?) if your job wasn't at risk - do you think (whether you knew it at the time or not - hindsight can be a great thing!) that they bought their house because they had agreed with dh that you would all move in? (I have a friend whose PIL moved in for 'a few weeks' after selling their house whilst they were looking for somewhere to buy. Months later they were still there. She was getting more and more desperate to get them out, turned out the dh had let them contribute a great chunk to the mortgage so they owned a chunk of the house and it had been their plan all along so they could live together, they just knew that if they told the friend that the dh wanted her and their dc to live with his parents she'd tell him no way - so they engineered it by stealth).
It's very telling that your dh is down with his parents - even if they are in a new house, as they are there it is still his 'home'. For you, it is a nightmare of a building site where you don't feel safe or welcome or even at home. And you are alone - he is with his parents. Which I think shows which side he is one - sorry.