That’s a whole series of independent questions, and I’m feeling rather tired, so I’m going to have to leave most of the details to other people.
The first big point is that while zigbee is the same everywhere, it also allows for different “profiles” including manufacturer proprietary profiles, and devices of different profiles cannot talk to each other. SmartThings uses the “zigbee home automation” profile, ZHA. So the first step is to find out exactly what profile the alert me devices are using. It’s not original issue, but there still might be incompatibility.
Next, smartthings is mostly a cloud – based system. Most actions require that you have an active Internet connection. The exception is the official “smart lighting” feature, and then only with some specific devices. So even if you can write custom code which allows you to integrate the alert me sensors with the smart things hub, just the fact that it’s custom code means that it will not be able to run locally.
Right now routines do not run locally, custom code does not run locally, you cannot arm or disarm the security portion of the system locally, and the mobile app cannot talk to the hub even if they are on the same LAN– – the app talks to the cloud and the cloud talks to the hub. ( technically, it doesn’t have to be that way, but the fact is it is that way in the smart things platform.)
If you are looking for a locally operated security system, SmartThings is probably not going to be a match for you. You can find much discussion of this in the forums.