Thank you all for the help!
I think it's going to be tough figuring out whats going on in the PCB in respect to both the hardware and software.
I was doing some research and stumbled upon the ADS1115 Differential Voltmeter, which measures differential voltage without needing to reference Arduino ground potential.
Have any of you used it? And do you think that will measure the voltage across the green and yellow lines without affecting the circuit?
Cheers,
B