This thing is the most important job you have in life to do. You MUST focus on it (during working hours) and when the day comes to a close and you can do no more, then that's when you stop.
When you have fought this, you can continue on in your life with DD in peace, but right now, you have a war to win. And you can win, he's shooting himself in the foot left right and centre. Keep asking questions, keep informing yourself. the more you know he's fucking all this up, the more calmly you will be able to be. that calm will help you think.
You don't want to have to face this, I get that, but you have no choice. You didn't do this, THEY did.
Thinking about the future with a nice guy is going to be what you aim for, and it will happen, but not while you still have this poisonous twerp and his disgusting family in it. Deal with them, get them where YOU want them, and make sure they stay there, and THEN you will have room in your life for someone that really is worth your energy.
As I've said, your rights here as a tenant have been violated, and you have been used by your ILs to perpetrate wrongdoing.
Good advice on the documents, keep them safe. If they have deducted rent straight from his cash, I am willing to bet that they are not declaring this income either. I'd be calling HMRC too when you are out and safely away from them.
NO to the contact, you said you would stick to Monday, so leave it at that.
Ignore the text, switch the phone off and go about your day.
You might want to get yourself a new phone/number and transfer everyone over to the new number, leaving him and his shitty people on the old number. It'll stop him having 24 hour access to you, and believe me, gaining that kind of mental space is priceless!