Nest thermostat HRV integration

I'm trying to assess the feasibility of a project I thought would be simple, but was wrong. First, I am completely new to micro controllers but have some hacking experience and am near the end of a project to integrate my Nest thermostats with a modulating boiler via a raspberry pi. That project (nest_odr) is on github but still a work in progress.

Now I want to setup one of my thermostats to control my Carrier HRV. I thought it would be a simple matter of attaching the thermostat to a couple of 24VAC relays to run the fan at low speed using the thermostat's fan timer function, and at high speed using the thermostat's dehumidifier function. But it isn't that simple. The HRV uses a wall controller and 4 wires to communicate with the control board on the HRV. The wall unit ultimate controls three relays in combination to control fan high and low speed, and damper to control fresh air vs. recirculating exchange. I have two options for controls:

Option 1: Brute Force. This option is to control the 120VAC fan and damper control wires via relays directly. It requires converting the 24VAC control voltage from the Nest to 12VDC to allow proper isolation of relays, but would basically duplicate the relays on the existing control board. There are two disadvantages of this option. One is that is involves household current which isn't a huge deal, but it's nice to do everything from the low voltage side if possible. The second is that it would defeat the unit's defrost cycle. The defrost cycle senses the incoming units air temperature and closes the damper to recirculate warm air to defrost the system as needed. The high voltage relays would be behind the unit's controller and therefore would keep the damper open regardless of temperature. So I will need to figure something out here if I go with this option.

Option 2: Low Voltage Control. I think a better option would be to create an interface that replicates the signals being sent from the wall controller. So if the Nest was calling for the Fan, the interface would call for low speed fresh air exchange. If it was calling for Dehumid, it would send a signal for high speed fresh air exchange. That way the unit's wall and on board controllers can still do their functions, including defrost. The biggest disadvantage of this option is my ignorance of micro controllers. I have no idea what is actually being sent across the 4 communication wires to signal the unit. Actually 3 wires because 1 is just a 12VDC supply. I've also been able to determine that the other wires operate at 5VDC with varying current depending on control function. But I have not been able to find any documentation that would help decipher what is happening there. It must be fairly standardized though because all of the HRV manufacturers seem to use the same controllers.

Option 2 appeals to me because it is more interesting for personal growth, it involves only low voltage signals, and it would probably allow me to replace the ugly wall controller all together. It would also allow adding more advanced controls like using humidity or occupancy sensors. So I'm looking for advice on whether this project is feasible for a noob like me. If so, any tips on at least figuring out the signal logic of the controller and how to replicate it with an Arduino.

Thanks in advance for any advice, even if that advice is to stick to my day job.

After more research, I think I'm starting to figure out the microcontroller/logic bit of this project and have more specific questions than my original rambling project description.

It appears my HRV is using serial (probably TTL) logic, possibly current modulated since my poking around with a multimeter indicated changes in current at different controller states. It seems my first challenge is to decipher the HRV wall controller signal logic to know what I need to replicate with the microcontroller. What is the best/ least expensive way to do that? Can this be done with an inexpensive logic probe? There are only 5 possible states for the controller:

Fan High, Damper Open Fan Low, Damper Open Fan High, Damper Closed Fan Low, Damper Closed Fan Off, Damper Closed

Can I methodically record what is happening on each of the two signal wires in each state with a logic probe to map out the logic? Or am I missing something? I'm pretty comfortable with switching logic but the more sophisticated digital logic is all new to me.

Also, initially I intend to add the thermostat based controller while leaving the original wall controller in place. This would add a second transmitter to the serial communication which I've read enough to know is very bad. Is there a good way to isolate two serial transmitters so they don't try to communicate with the receiver at the same time?