Unfortunately, these are exactly the two features that most security systems do not expose to third-party systems. Including IFTTT, Alexa routines, even HomeKit. In particular, they won’t allow you to disarm from an outside system as that would be considered a huge security hole for any system which was not itself UL certified as a security system.
so…
IF THE ALARM GOES OFF
To tell if the alarm went off, you normally have to treat it as a physical event and then try to capture recognition of that physical event through something that communicates with smartthings.
That could be a relay wired into the security system. It could be an acoustic sensor that hears the siren go off. But nothing super simple.
A number of people have use the wired relay method, so I’ll leave it to those who have actually done so to comment on that. But you can do it with Konnected.Io and Noonlight, for example.
It’s often easier to trigger from a text notification from the security system, and you can do that with IFTTT, for example.
ARM/DISARM
There’s no good software way to arm or disarm any of the UL listed security systems. The one thing you can do is use a “robot finger“ to physically push a button on a key fob or keypad. The security system won’t know the difference between that and a person pushing the button, so it will accept it. Switchbot is a Device of this type which does have a smartthings Integration, although you do have to get their mini hub in order to get the integration. These are nice little devices and I do use them in my own home, but Integration is Cloud dependent so I don’t know if you really want that for arming or disarming. But you could do it.
here’s a review I wrote of the devices last year if you’re interested in that option.
2020 Switchbot Review ( robot button pusher), integration through IFTTT or SmartThings app
There are a few security systems where you can get some integration through Alexa by directing an echo device to speak the security system command to a second echo device. You’ll also need a pin code at least to disarm. This is doable, but pretty Hacky and it does mean that anyone who’s in your house will hear your security pin. So while it’s doable, I don’t know of anyone actually using this method.
NEW PLATFORM
Also remember that smartthings is in the middle of transitioning to an entirely new platform, so anything written in groovy will no longer work once that transition is complete unless you find a way to host it yourself. So before going all in on any custom solution, make sure you find out whether the authors intend to support it for the new platform or not.