My Mom's Ex-husband Is Still A Co-owner On Her Mortgage. He Walked Into Her Home Uninvited, Screamed At Her And Her Partner Through A Bedroom Door, And Later Told Her He Planned To "blow His Brains Out" In Front Of Them. Police Said They Can't Act Because His Name Is On The Loan. What Are Our Option

Background

This will probably be long-winded, as family matters usually are, so I apologize in advance. I'll try to explain it clearly.

I (F23) live with my husband (M28) and our 5-month-old baby in the ground-floor area of my mom's house. We contribute money to the household but have no formal lease. We both have mail here and list it as our primary residence.

My mom, my little brother (now 15), my stepfather at the time (her then-husband), and I moved into this house around 2019-2020. They took out two mortgages, one of them a VA loan from her then-husband, and both have both of their names on them. They separated about two years ago. My mom looked into taking the loans into only her name but couldn't afford the payments because of her credit. From what she's told me, their divorce paperwork explicitly says she can stay in the house with the current mortgage until my brother turns 18, and then she has to either sell or take the loans into her name. I will say I haven't actually seen this paperwork myself.

The incident

Yesterday evening, my husband and I were in our living room area with our baby and a friend, waiting for the rest of our friends to come over for DnD night. We'd left the garage door open and the side door unlocked so they could get in. My mom had her current situationship over, and they were upstairs in her bedroom. His car was in the driveway.

I went to the bathroom, and while I was in there, my mom's ex-husband walked in through the garage and up the stairs, leaving both doors wide open and not announcing himself. After a moment, my husband got up to check, since our friends usually say hi or come into our area. He noticed the open doors, looked outside, and saw a truck he didn't recognize. He had our friend hold the baby and went upstairs, having figured out who it was. (The ex has openly admitted to parking on the block at night and watching the house to make sure my mom doesn't have anyone over.)

When my husband got upstairs, the ex was standing outside my mom's bedroom door, screaming at them through it. The conversation went roughly like this (heavily paraphrased):

Husband: What's up, my guy?

Ex: Your mother-in-law is fucking cheating on me.

Husband: Uhhh, aren't y'all divorced?

Ex: We were, but we got back together. (Not true as far as I know. They've hooked up off and on, but he hasn't been in the house, and he'd explicitly said he never wanted to step foot in here again.)

Husband: You should probably calm down or leave, buddy.

Ex: You need to go back downstairs, this is my fucking house.

Husband: Do you need a hug or something?

Ex: (leaves out the front door)

My husband then came and got me and gave me a rundown. After a few hours, I convinced my mom to call the non-emergency line to get this documented in case he came back. He did, in fact, circle the block at least twice, according to our friend, who went out and chain-smoked in the driveway to keep an eye out.

My mom got called into work, so my husband ended up talking to the police and sending them her way. Today she told us there was no report to file and no grounds for a restraining order because his name is on the loan.

She also talked to him on the phone for about three hours afterward, trying to deescalate. He told her his plan was to come into her room and "blow his brains out" in front of her and the guy she was with. This isn't the first time he's threatened suicide to pressure her, usually to guilt her into hanging out with him and eventually sleeping with him.

Where we are now

I'm terrified for my mom's safety and especially my baby's. This man is clearly unstable and could be dangerous, and I don't know what our options are besides moving out, which we can't afford. My mom doesn't seem to grasp how serious this is.

A friend is lending us a firearm for protection, but I do not want my husband to be the one who ends up facing charges if he comes in again. I'm asking specifically how to protect ourselves without getting into legal trouble.

My questions

Can a co-owner on the mortgage be issued a trespass notice or be legally barred from entering the home?

Can my mom get a protective order in South Dakota without a history of physical violence, based on the stalking-type behavior and the threat?

What's our legal status as non-lease occupants if this escalates?

If he enters again, what can we legally do (and not do) to protect ourselves?

Should she go back to the police, and what should she say so the threat gets documented?

Location: Sioux falls, SD

Edit: there are lots of good advice and questions that I just honestly don't know the answer to so I'm sending this post to my mom so she can get information directly from the source. I'll talk to her later and hopefully relay any information she'd like to share/reply to the applicable comments.