Let me see if I can shed some light on your questions.
This is really the ST platform having issues. Whenever a Scheduled Event for polling fails to run then it is only when/if one of the watchdogs that I’ve put in place that it is able to recover. There are a few things that will trigger the watchdog: the authentication token scheduled event, the watchdog scheduled event, sunrise, sunset or a “manual” poll event. Those are the only things (right now) that can trigger the watchdog (I use manual in quotes since it could also come from an outside event such as an IFTTT type trigger, Pollster or even Rule Machine).
I could try to get more aggressive on the watchdog events and increase the frequency, but I don’t think that will help much since the watchdog scheduled event also frequently fails. So the scheduled events all kinda depend on each other to get them back on track. Having one fail too quickly would likely mean that it won’t ever recover.
From everything I’ve read so far in the forums, doing any kind of polling faster than 5 minutes is likely to cause trouble. And using a Scheduled Event to facilitate that likely won’t work for very long. If you really want to try it, however, you can use Pollster to give it a try (https://community.smartthings.com/t/pollster-a-smartthings-polling-daemon/3447). You will note, though, that even the Pollster thread warns against polling faster than 5 minutes.
A “poll” to the Ecobee API is really just one request that returns everything, so polling for just motion and temperature doesn’t really change anything.
What I have been considering, is allowing for other outside events to be used to trigger a poll rather than just using the scheduled events. This would allow for more frequent polls or to poll immediately on certain actions. I could either add that to my code or you can actually already do it today using Rule Machine. Just pick some sensor you have that changes based on an event: a switch is turned on (or off), a motion detector is tripped, a door is opened, a temperature changes, etc. It could be any combination of them. These event based triggers can also allow the system to recover more quickly if a scheduled event fails.
But at the end of the day, if you really want real-time motion type control then the Ecobee sensors will never give you that. They have a delay on providing their status and once on they are active for 30 minutes.
There is possibly some hope coming in the future as Ecobee is testing a push API that could send us events if something changes. Unfortunately, they have not published that API so I don’t know how easy it will be to integrate yet.