>> The protections diodes are documented in the manual
In that case no need to provide your own.
>> The board is at the transformer. The long wires to the controller carry the DC output.
That is the best way to do it
>> I agree with you about simplicity, but one of the goals of this project was to be minimally invasive. The unit is still under warranty and I didn’t want to void it (possibly) by making modifications.
If all you want to know is when the compressor is running you have simpler and cheaper solutions. As I mentioned, I am sure you can esily find access to install a relay in parallel with the compressor without voiding any warranty. But you can also install a relay in the cable that feeds the unit. Let me explain. You can get a relay that will activate at (say) 5 amps and the voltage drop is near zero. You insert in series with one of the wires and it will close when the compressor is running but the fan will not use enough to close the contacts. You can make a short “extension cord” where you insert the relay so there’s no need to do anything to the A/C unit itself. This would have been simpler and cheaper. Your solution makes sense if you need to measure the Amps, if you want a value, not just a yes/no signal.