Afternoon - I've been away so apologies for not checking in, I did wonder how this played out. Get yourself a 
because this turned into a long one.
The more you say OP, the clearer it is that your situation is complex but also that you are incredibly vulnerable and I am sorry to say it but it might even be that your partner targeted you because of this. If he knew about the PTSD and the lung problems, not to mention a lack of self esteem and probably a mistrust of your own mind at times, you are an ideal target for exploitation. I am sorry to put it like that because you are so much more than that and clearly stronger than you think, but I think he knew that he could manipulate you because you were so busy dealing with multiple issues that your defences were aready stretched thin.
I think you need to start giving yourself credit for just how much you are managing to take and how you still have the willpower and the mental clarity to know that this is not right.
His mum sounds like a fucking prize too. I'd guess that she actually knows that you pay everything and doesn't care, as long as her son is golden. She's happy to have you as some sort of family dogsbody and I say again, you are worth so much more, especially to your own mum and your children.
I stand by everything I said before: keep a diary, write it all down (past and present), talk to your mum, talk to Women's Aid, speak to your GP, the CAB if possible, and to the kid's school.
Read Why Does He Do That (free PDF version).
Set up the new (secret) gmail account don't use the gmail on your phone/shared laptop/tablet, he won't even know it exists.
As for the bank account, I read into the code of practice for protecting victims of financial abuse and there is good news! You can find a list of highstreet banks that are signed up to it here and the text is here. Essentially, you can disclose financial abuse to your advisor and they can take discreet steps to protect your from him. In your own private, secret account, you can use your mum's address, but if you inform your main bank (the app to which he hass access) they can do things like communicate with you separately from him, defer decision making, request separate signatures to documents so that if you are being led/pressured into a financial decision that you don't want, you can tell the advisor in privacy and they can create an admin SNAFU to block the decision.
All considered but with especially the finances in mind - DO NOT MARRY HIM. Use any excuse, Covid-19, your lungs, have your mum fake a fainting fit, whatever, to push the wedding back (and again, tell him she cannot afford to contribute but put anythig she can give ou into your secret bank account).
I have no experience of SS so I cannot pretend to know how that feels, but if you are taking all the other steps to spek to the GP, the school, separate finances, go on Entitled To, etc., surely these are all things that would demonstrate that you are compis mentis and able to care for your children? Others can advise on that experience but I honestly think that taking action and meeting those concerns head on might be for the best.
Also, those dislocations sound disturbing. Ok, you have hypermobility, but how bad was this nightmare and how hard was he holding you or were you thrashing to dislocate a shoulder?? Just the thought of that made me wince. You arethe only one able to truly judge that situation, but it does not sound good, even with a later diagnosis.
In order to end on a positive note, just remember that there are plenty of people here rooting for you, and hundreds of women on mumsnet who have left partners in terrible fanancial/physical/emotional states and still felt better for it and made better lives for themselves and their children.
You don't have to do everything today and you don't have to be out by next Tuesday, but please start takingthe baby steps that will build your confidence and your independence for the day that you truly decide you've had enough. xxxx