[clubjoker]

I've also today received the step-down buck converter, set it to 9.0V and tested with that supplying the Pro2+ RX. Same issues with the regulators running hot:

I really don't know why your voltage regulators are running so hot, but as you are using a step-down buck converter you can try lower the output voltage to around 8V and see if it makes any diference. Doing it will not affect the functionality of the system.

I will try that, reducing the voltage to 8V.

But I would like to find and fix the root cause of the apparently excessive power draw. I wonder if there is some static electricity damage.

From the diagnostics tests I ran previously, and the fact that the TFT display is working fine, can we be sure that the 3.3V<->5V buffer level-shifter 74AHC244 chip is fine / not damaged? I did remove the power diode on the Meduino before powering the system up, but wonder if chips / transistors might have been damaged by static electricity or (less likely?) by overheating when I soldered.

Is it worth me trying by replacing the BMP280 sensor with a BMP180 sensor I have spare? (Pressure readings seem to work OK, but it would be good to eliminate that as the cause of the problem).

Intending to minimise the risk of future failure, I have soldered all the components except the RTC to the PCB, so it's a bit of a pain to swap things out to test them. Maybe I would be best to try to build another PCB up, but $$$