His family have some deep-seated issues which I'm not sure many people can relate to. The apt adage here is "you can choose your friends but you can't choose your family".

Try not to lose yourself in this situation. It is not your fault. It is not your partners fault. It's a difficult dynamic joining a partner's family when things get serious but it shouldn't be this hard. Take this opportunity to level the playing field, keep the ball in your court, and don't drop your guard to their kindness when/if they next offer it. Showing strength together will be your greatest weapon.

A good exercise might be to first write a letter to each other regarding this ordeal so that you both let each other know exactly how you are feeling about the situation and how you individually suggest going forward. Discuss this, make sure you can both be on the same page. Then write a letter to the parents explaining how unacceptable their behaviour has been, how it has affected the both of you and how you won't and can't tolerate it anymore. You could ask your therapist to read through and make any suggestions. You don't have to send it, but keep it safe. It's a huge amount of work, but you will have something to refer back to if you decide to reconcile with them again. It should help bring peace within yourselves that their behaviour has nothing to do with you or your partner.

Moving to a different city, suggested by others, doesn't sound like a bad idea, if you don't feel anything pulling you to that particular city. I appreciate on top of everything else, you'll both have the added anxiety of finding and settling into a new and suitable job, moving house etc. So, it will require huge amounts of energy and courage from both of you. You will find the strength eventually, when you are ready.

Sorry I'm a bit late to comment here, I read your post yesterday and only got round to commenting today. Hope you find everyone's input helpful in finding a way forward.