This is a horrific situation to be in, but you are handling it brilliantly.
For now, I think it’s best to write off his family. After the divorce, you’ll see who sticks around—some may be unaware of what’s happening, while others may be complicit. The divorce process may reveal their true positions, but that’s not your concern. The most important thing is to get yourself far away from these toxic people.
Have you spoken to your support worker and/or solicitor about financial abuse as part of the divorce proceedings? Some of this may help with regards security for your housing situation. This article may be helpful: Economic Abuse in Divorce.
Are you seeing a Family solicitor I.e, a specialist in divorce? I’d look for one that has expertise or can access expertise on financial abuse and I’d push for a forensic accountant as they are a specialist, don’t rely on the solicitor alone. They’re up to £500 a hour but can help track down hidden assets and check financial irregularities you (and your solicitor) won’t have access to.
He may be storing documents online rather than keeping physical copies, or he could be hiding paperwork elsewhere, at his families houses for example. Do you have access to his laptop, iPad, or other devices? If so, it might be worth checking for anything you can find - browser history, account login, banks that are not his usual etc.
Keep your powder absolutely dry until you have all of the information you need, and then file. Do not warn him you are going to do this, stealth and surprise are your biggest weapons here. Hard as it is, try to stay ‘as normal as possible’ with him. He does not value you and likely therefore will fundamentally underestimate you - use that to your advantage. Well done for telling family and friends but make sure they don’t say anything yet either.