Hi,
The op amp is receiving set voltages from the two voltage dividers, but the positive is receiving additional variable voltage from the NTC.
Looks like that can be switched in or out by the mosfet and then one of the other two mosfets switch on and apply their set voltage, eg frost stat or night temp.
You want to measure the output voltage at point ADC and see what the voltage is when each mosfet is turned on, one at a time.
Assuming they are true logic level mosfets, " from MCU I/O" then you can apply +5v to their gate via a 1k approx resistor..
If you ca,n check that the MCU is providing a voltage to the mosfet gate.
You need to show the 2700ohm divider , and how hot is the 7805, hold you finger on it for how many seconds ? just pcb copper for the heatsink ?