Figured out the Controller!!! This is how you can have fan (s) on at slow speed 100% duty for air-swap/temp lights off, and then kick the fan up to 100% if EITHER the temp OR Relative Humidity goes out of range.
Fan Low-Speed Controller (FLSC): This guy right below will be used to set the “Always ON” speed for the fan:
This little guy lets you control speed with the rheostat, and you can set the voltage range with that little trimmer screw on the blue trimmer. 
Low-Speed Interrupt Relay (LSIR): In the HOT wire on the OUTPUT side of the FLSC (above), we insert this Normally Closed relay (120 VAC ctrl voltage, </= 240 VAC switched voltage):
Temperature/Humidity Controller (THC): The controller below will allow basically ANY setpoint temperature/humidity and hysteresis level to be set.
Powered by 120 VAC. When the temp/RH exceeds the setpoint, it triggers the respective power out relay (120 VAC OUT) to whatever you have hooked to it. Simply connect BOTH the relay outputs to the ventilation fan AND the LSIC INPUT. (Does not matter if both are on at same time, as they are both powered at same voltage from same source.)
In THIS way, when the fans are kicked up to 100% to address a high temp and/or RH, the power out to the fan(s) ALSO activates (opens) the LSIR, which cuts the LSFC (at < 120 vac) out of the circuit. Thus, the fans speed up, but no reverse current shows up and smokes the FLSC or does other bad things.
When the temp and/or RH are back in range, the THC cuts the power to the fan AND the LSIR. The LSIR closes and powers the fan BACK on instantly at the lower voltage… switching is so fast that it will just look like the fan is slowing down. 
Not bad for a TOTAL of $33.43
Oh… and I figured out how to do it with ONE fan in the attic and no down tube inside closet to get supply air from one room to the next. Will put that in a separate post.