I'm sorry your Mom is having such a bad time. It's such an old and familiar story. I'm shocked that the local police are so unhelpful though. Police pretty much everywhere (in Canada at least) have learned to take DV very, very seriously, and most have training in dealing with situations where victims are in danger. Some things they can't do much about (such as retrieving the cat), but if there is a court order for someone to retrieve their belongings, they will have assistance in retrieving them safely.

Even with police on your side, some dtermined abusers will manage to do further harm, so please be very careful.