Welcome, Jason, to the OEM forum.
I know next to nothing about the Heatpump Monitor, however I’ve had a look at the drawings and the what I think is the core section of the software.
First, the easy one: The energy monitoring part is designed to accept a nominal 9 V a.c. signal. The type of transformer used is always specified at full load, whereas in this application the load is almost non-existent, so the actual voltage that it expects to see is around 10 - 12 V. If you can change resistors on the circuit board, then the actual minimum voltage is about 1 V (rms), anything above that up to the voltage rating of the connector (probably about 25 V without checking) could be used with the appropriate resistor change. However, to measure real power accurately, the transformer needs to have a relatively undistorted output and it’s not referenced to GND, so sharing the voltage from some other equipment could give you problems, either with the heat pump monitor or with the other gear, or both. You definitely cannot use the same transformer for both power and voltage sensing.
Looking at all the source files I can find, the RFM module (the 433 MHz ISM Band) is disabled in software - the relevant line is
#define RFM69_ENABLE 0
in all but “HeatpumpMonitor_AutoTemp”, where it is enabled (1). To disable it in there, you need to change that to RFM69_ENABLE 0; save the file, recompile and reload it.
If this doesn’t help, post again and I’ll find someone who knows for you.