That whole situation sucks. I lived for a time in Pomona Drazzil, you're in SB right? So you know I know what you're talking about.
Only things I can suggest is: Do you really want a car alarm? If they want to flatten the tires, is a car alarm gonna stop them? I mean, if it goes off at 2 am, what do you see yourself doing about it? I'm NOT being a smartass, I'm asking you to really consider it. A)you go out and confront the punks=obvious risks as you should assume they are armed. B)you call the cops=digging yourself a bigger hole maybe.
They might see you getting a car alarm as a challenge, a call to f with you further. The idea is to stay under their radar as much as you can, not call attention to yourself.
What I'd think about: securing the car itself from being stolen, that's the best you can do. A brake lock maybe, plus a killswitch somewhere. Same with your house/apartment- secure the doors and window with bars maybe? So at least your mom is safe inside, when you're not around.
As far as the cops, I'd avoid 911 except for emergencies, if they are shooting AT you for example, THEN call cops. For what you want to do, ie tell the cops what's going on but stay undercover, you need to talk to a cop in person(obviously away from home, at the station maybe) or call the station number and talk to someone that way. If you call 911, you can't count on the kind of discretion you need.
Also, they may think your MOM called the cops, not you. That's something they won't like, but it's not as bad as if they thought you did it. If confronted, play it off like that- "Some f'head was shooting up the street and the old lady got scared, whattya expect?", that kind of thing. They're much more likely to retaliate if they think you did it than if they think you're mom did it.
If I think of anything else, I'll post it.