I've been on your shoes, it's bloody tough. Firstly, the sectioning, you need to forget how bad it looked, it was for his own good. No one gets sectioned unless they're extremely unwell.
If he stops his medications, his behaviour will take a downward turn, he will be sectioned again. If he is admitted to hospital, you must refuse for him to return to your mothers. Be absolutely adamant that the situation is not sustainable, do not agree to even one day back at your mums.
Now, what ended up happening in my family, we had to get a court injunction to remove him from my mums house, my mum didn't speak to me for months but then finally conceded that it was the right thing to do. She was being abused abs if this was her husband, she would've divorced him and it was not right that her DS was doing it. It was not helping DB to have these rages, it was making his illness worse and it was making my mum ill. He continued to phone my mum, which was fine as she could put the phone down if he started shouting etc.
He got housed as he was now homeless and a vulnerable adult.
It's really hard work and your mum needs to understand that to help him she must do this. Once he's out of the house and not torturing her, which is what he's doing, the sleep deprivation through hours of interrogation is torture, her health will improve.
Your DB will cope with living alone, he may get an assisted living home, if that's required.
Services are very stretched and you're going to have to keep strong about what's required.
I was appointed "closest relative", not next of kin as that was my DM. So I dealt with hospital discharges and I could call for a mental health assessment (which always led to a sectioning), if I felt necessary.
My DB died fairly young, he was having an episode and two days before his death I managed to get him to agree to be admitted to a psychiatric ward, he spent 36 hours stuck in a&e then he got transferred, he then suffered a massive heart attack and couldn't be saved. My DM had already passed away by this time.
Honestly making sure he lived away from DM was the best thing for both of them, but a lot of heartache at the time..
Good luck.