Need to know the thermostat? is it a WiFi, a digital or a old mechanical. Thermostats will have a strap option or a program setting for the type of heat. Either fossil fuel (gas or Oil) or Electric heat. Many thermostats control the blower fan to give the results you're looking for. Thought the manual states its a non programmable setting within the furnace design as how long the fan motor starts and ends for each heat cycle.
Your very first question is No, ECM motor speeds are controlled with a low voltage, usually 0-10 volts on one signal lead. You stated that the 24 volts ac goes away when heat cycle ends and the fan motor stops.