Mitsubishi remote thermostat (which is now plugged in via the spare port on the dongle)

The Mitsi remote thermostat should not be plugged into the dongle, that should remain connected to the CNRF port located just above the CN105. Therefore the Thermostat should remain unaffected.

It was likely the MELCloud dongle you mean which was consuming the CN105 port

dongle just a directly wired parallel connection

The Serial communication that FTC speaks over then CN105 would not support multiple devices speaking on the same Tx/Rx lines, clashes, confusion and failure would occur eventually.

So the adapter has act as an active negotiator of communication between itself, the FTC and optionally, MELCloud.

Thankfully the chance of failure is very low. There is the possibility for the WiFi version to recover via USB first with the GitHub instructions and open source firmware and failing that, the controller is also a pluggable replacement.

On the issue of what happens if the dongle dies, presumably my existing Mitsubishi remote thermostat (which is now plugged in via the spare port on the dongle) will be offline, so won't work unless I plug it in directly onto the FTC6?

Or, is the extra port on the dongle just a directly wired parallel connection, so the Mitsubishi remote will still work?

There is an excellent write up here:

https://renewableheatinghub.co.uk/forums/renewable-heating-air-source-heap-pumps-ashps/configuring-third-party-dongle-for-ecodan-local-control/paged/2/#post-54561

Does "publishing" json via mqtt write data permanently to the esp device, or is it just in RAM so it gets lost if there's a power cut?

What happens to the heatpump if the dongle or my HA server dies? Can I simply switch the FTC6 back to WC mode and it carries on like before?

On these specific questions:

1. Publishing WC will subsequently be saved to the nonvolatile storage, so it will be retained on power loss. This is the case for most parameters, unless there was a specific reason not to

2. Well if your HA dies, the dongle carries on with the saved curve. If the dongle dies then it will remain in fixed flow. In both cases you can swap your settings via the main controller as it overrules in the Ecodan design 

Since my basic WC curve works fine on a normal overcast day, I think that I need:

the "temperature offset" function to be automated, triggered by a certain differential between the hallway and (unheated, but massive solar gain) conservatory Tapo temperature sensors.

The main thermostat (the mitsubishi remote) to be lowered if the above temperature differential exists, and I have opened the door from the kitchen to the conservatory, to effectively turn off the heating.

Reading through the documentation, I will need to "onboard" the WC curve, define it the same as my existing curve, and put the heatpump into fixed flow temp mode?

This is where it starts to go over my head somewhat.

I've done a bit with esphome to create my own devices, but mostly by copying/pasting/modifying existing code, so I'll probably make it work somehow - once I get my head around the basics.

Does "publishing" json via mqtt write data permanently to the esp device, or is it just in RAM so it gets lost if there's a power cut?

What happens to the heatpump if the dongle or my HA server dies? Can I simply switch the FTC6 back to WC mode and it carries on like before?

Powered up the FTC6, and was a bit surprised that the heatpump fired right up, despite the Mitsubishi remote stat being set to 19 degrees?

The default behaviour after any power loss is the Ecodan will run a hot water cycle - i suspect it was this which started.

Powered up the FTC6, and was a bit surprised that the heatpump fired right up, despite the Mitsubishi remote stat being set to 19 degrees?

I connected my iPhone to the "Mitsubishi Bridge AP", entered my WiFi details. I left everything else at the default, and clicked "save".

After a minute, the heatpump took notice of the remote controller, and stopped heating.

Is it normal that the FTC ignores the remote stat until the dongle comes properly online?

Looking at the LED on the dongle, I can't really tell if it's Orange or Green - looks Green dead ahead, but Orange from the side.

Where can I find detailed instructions of what to do next?

Edit:

I was using the wrong MQTT credentials!

When I first used MQTT to work with my Sofabaton X2 remote, I followed a guide that said it was best not to use the default credentials, but to set up a user just for MQTT.

I've re-configured the dongle with the correct MQTT credentials, and the LED is now green from whatever angle I look at it.

I can also see a load of stuff in the MQTT integration section, so it all looks good.

While @F1p's dongles are great, the FTC can do this itself. It's called Summer Mode and can be found in the Service settings (default password is 0000) then Operation Settings. Summer Mode has two ambient temperature settings (on and off temperatures) and two "judgment times" (how often to check the ambient temperature - 1 hour works well for both).

Can you confirm that it comes pre-flashed and ready to plug in,

Yes, Pre-flashed plug & play