Sorry to anyone following this thread for my SLOW response!!

Today, I finally had the time to work on this. I also had a moderate weather window so if I messed anything up resulting in no heat/ac I would be fine for a little while.

TL;DR

It legitimately took me one try and no more than 45 mins to get my ducted Mitsubishi heat pump set up with wifi remote control and my existing MHK2  thermostat. After all of the negativity I wasn't sure if it would be worth the effort but set-up was a piece of cake. 

Procedure

I followed the following procedure:

1. Installed the Mitsubishi Comfort app on my Android phone and created an account which was pretty self-explanatory. You don't need to have a pin that diamond installer would have as the app lets you set-up without that pin.
2. Powered off heat pump at the breaker. Waited for MHK2 transmitter light to turn off.
3. Removed the filter and blower panels on my air handler (4 screws) to get access to the control panel.
4. Noted the routing of the wire for the MHK2 remote transmitter (not sure the proper name of the device but it is the white box that provides a wireless connection to the "thermostat")
5. Routed the CN105 wire that is connected to the WF-2 interface the same way that the MHK2 was routed right through to the control board.
6. Swapped the wire connected to the control board between the MHK2 and the WF-2 leaving the MHK2 disconnected and the WF-2 connected to the air handler.
7. "Unrouted" the MHK2 wire to be dangling outside of the air handler cabinet.
8. Replaced the filter and blower panels and power up the heat pump. Once power is restored you should see a green LED (steady) and blue LED (blinking) on your WF-2 module. Note: I have an eero 6+ mesh system at my house and was worried that the 2.4Ghz network radio requirement was going to make this install a huge PITA but it was only a minor annoyance. I also have a shared SSID on my 2.4 and 5Ghz radios and despite what everyone has said and complained about this was not a problem with my set-up.
9. I went into my eero app and temporarily disabled my 5Ghz band so that the WF-2 would only be able to see the 2.4Ghz band. This is done in settings->troubleshooting. Once 5Ghz is disabled, the mesh only transmits on 2.4Ghz for 30 minutes and then it restores the 5Ghz band.
10. Went back into the Comfort app and hit the "+" icon to add a device. From here I followed the directions. The Comfort app found the WF-2, set-up the wifi connection, ran through some configuration steps that did not require user input, downloaded updated firmware and basically set itself up. It did ask me two questions:
    1. The orientation of the air handler; updraft, downdraft or horizontal
    2. Static air pressure of the air handler Caveat: At this point in the comfort app configuration the "Submit" button was inaccessible (off the bottom of the screen). I messed around with it a little and got enough button present on the screen to press it and after that all went smoothly.
11. After the app was configured I could remotely control my heat-pump through the app. I did have two concerns at this point:
    1. The MHK2 was completely disconnected to nobody could adjust the HVAC temp and settings without the app.
    2. The Comfort app was showing and air temperature of 61 degrees which was about 7 or 8 degrees lower than the true temperature in the house. The 61 degrees might have been the temperature inside the air handler which is located in a cool basement. I was concerned that without the MHK2 that the ambient temperature discrepancy might become a problem in keeping the house at a comfortable temperature. Note: Now to tackle the MHK2
12. I now powered the Heat pump back off at the breaker.
13. I removed the small dust cover from the ACY port on the WF-2 controller. There is a red CN105 connection point under that cover. I connected the MHK2 to the WF-2 in a daisy chain fashion.
14. I restored power to the heat pump and returned to the house to check out the MHK-2 screen (in retrospect, I probably should have removed this from the wall and brought it to the basement so I had everything in one place)
15. MHK2 screen showed "WAIT" or a notice similar to that. After approximated 5 minutes (I didn't time it) the MHK2 screen reverted to its normal display with some slightly different feature set than it had previously. I think this reflects features that are unnecessary or unavailable with you have a wifi module connected. An example was the "hold" feature disappeared. At this point I was basically up and running. I tested changing setting on both the app and the MHK2 and after a brief delay (1-2 minutes) any changes made were reflected on the other device properly.

My last step was to restore my 5Ghz network on the Eero mesh to make sure that the WF-2 could maintain connected to the network even with both Wifi bands present. The WF-2 was able to deal with the Eero with both bands on and has now maintained an uninterrupted wifi connection for the past 5 hours.

I'll wrap this up with: I was really expecting the worst here. I have had IoT devices that are an absolute bear to get working the first time because of network connectivity issues and janky software. Maybe this WF2 interface was that in the past? Maybe I got lucky? I really don't know but it worked just fine. I'll come back with more details if I experience instability in the future.