Can I use the ADC-S2000-T-RA temperature sensor to trigger automations? For example, if the temperature rises above 78 degrees, active a Z-wave switch that controls my room A/C unit.
No, the Remote temp sensor is used in conjunction with the T2000 for temp adjustment rules specifically.
Are there any devices out there, other than the T-2000 thermostat, that will report both temperature and humidity to alarm.com? This room is not wired for a thermostat.
No, you would need to use a thermostat for this purpose. A thermostat is nothing more than a logic relay board though, and could likely be used for your purpose.
If not, as a last resort, is there any reason I couldn’t just put a freestanding T-2000 in this room just to report temperature/humidity readings and trigger automations? Note that I already have 3 thermostats active on my account, so this would be the 4th (not sure if there is a limit on these).
Some panels have a thermostat limit, like the Qolsys IQ panel and IQ Panel 2, but you wouldn’t hit it yet. There is no limit with ADC, but they recommend no more than 5. The max with the IQ Panel 2 is 6, though this is adjustable and by default this is in fact set to 3. You just need to change it in programming under Settings - Advanced - Installation - Devices - Z-wave - Z-wave Settings.
Note that if you just use the thermostat on battery power you will need better Z-wave communication to that location in the home as the Tstat would be operating in power save mode. You could use a 24VAC plug-in transformer to deliver AC power if desired instead.
If your freestanding AC can be controlled by low voltage trigger then you would be able to use the T2000 to control it.
If it can’t and you are instead looking to control it via a Z-wave Plug-In Appliance module or the like, the thermostat cannot directly trigger a switch rule like that. You would need to somehow use a relay to trip a sensor and trigger the switch by sensor activity.
This could use literally any thermostat, it would not require a Z-wave one, as the thermostat would simply trip a relay which opens the sensor circuit. You would then trigger the Appliance Module by the sensor activity for a period of time.
In general I don’t really recommend this work-around though. The sensor work-around can be implemented without trouble, but the likelihood for issues is much higher than directly using a Tstat to control the AC if possible.
Lastly, pretty much any freestanding AC or Heater should have its own built in thermostat and internally regulate when it is turned on and off, except perhaps an electric space heater. Do none of these devices have any sort of auto function?