Is it possible to install electrical meter between main panel and subpanel?
It is certainly possible to insert a meter, just by adding a meter pan and buying a meter (the utility does not need to be involved - it's not a utility meter, it's local to allow you to correctly distribute the cost unfairly applied to the end-unit owners in the present scheme.) Used &quot;dumb&quot; mechanical meters should be widely available since utilities replaced a lot of them with &quot;smart&quot; meters in recent years. Since it's multi-unit housing, a licensed electrician is required, and the fire alarm company needs to know when it's happening, but that should be about it - don't know if it needs a permit, but that's a LAHJ call in any case, so ask yours (or your electrician - NOT handyman.)
The 1500W heaters probably relate to the other question's room temperature sensors - there's your heating element to keep things from freezing. And probably most of your operating expense. Or the sensors may just be for alarm purposes, with the heaters on a normal thermostat.
The breakers will trip if they need to. The device on the handle prevents them from being manually turned off. They will trip regardless of the handle position, if they need to trip.
As noted many times in subpanel questions here, the sub-panel main is a local shutoff, and does not need to match the feed breaker supplying the sub-panel. So a 20A feed to a 100A sub-panel main is perfectly legit.