First, which brand(s) of thermostats are you using? This will determine if your need ScheduleTstatZones (for any ST connected thermostat with standard capabilties) or ecobeeSetZoneWithScheudule (specially designed for ecobee thermostats)
Second, yes,the above smartapps can indirectly use your presence sensors when they trigger the ST hello/location mode (ex. Away, Home, Night). This can be done when you configure the appropriate ST routine in the ST mobile app.
About your requirements:
1- When you say "Follow the schedule when people are home", you mean the schedule in the smartapp or the schedule at the thermostat unit?
The smartapps will follow their own schedules, so you need to define a Home schedule within the chosen smartapp and associate it to the ST hello mode “Home”.
3- The same for "Ignore the schedule when away and default to a specified away temp"
The smartapps will follow their own schedules, so you need to define the “Away” schedule within the chosen smartapp and associate it to the ST hello mode “Away”.
4- Use IFTTT/Google Home to manually kickoff the sleeping temperature (ie, Google, I’m going to bed)
This will be done by triggering the right ST routine. The smartapp is not involved here except that the schedule associated to the ST hello mode (ex. Night) will be trigerred after.
The way the smartapps are designed is that:
- You create & configure all your thermostat schedules (with start/end times) within the smartapp for Home, Away, Night , any other custom ones…
- You may (or not) associate a given schedule to your location modes (Home, Away, Night, etc.)