Hi OP, I've namechanged as I have similar personal experience and it may be outing.
My DF was the same, and unfortunately his mental health problems (which included hoarding, paranoia etc) were the cause of significant volatility and domestic violence in my childhood home.
If your DB can, please offer your DM space to get away and get better. Your DF is somewhat capable of doing things for himself (you mentioned he drives etc), so let him get on for a while or drop off meals etc.
Can I ask how he has the money to buy stuff? Is it his pension/from a shared bank account etc?
You must speak to Social Services to help both your parents get support. Make sure your DF knows it was you/your DB who has done this, rather than allowing him to shift blame to your DM.
I really feel for you because hoarding is such a secret shame, for everyone. I knew my DF didn't want to live they way he did - it was a compulsion, and absolutely linked to struggles in his earlier years and lack of control. I also know how suffocating and awful it was to step back into the family home, having to remind yourself of that existence and how your DP live.
Please try and take this opportunity with your DB to get your parents the help they need.