For grounding the cold pins, could I wire together all the cold pins to a single wire that attaches to the green pin highlighted above?

Regarding output level (and maybe grounding too) I should note that these signals will be sent to MIDI Guitar which is a program that maps guitar signal to MIDI, so the tone of the signal doesn’t matter, it just has to be intelligible by the software. For gain I could use a THRU algorithm on the H9000 to apply gain, or maybe just digital gain in MIDI Guitar.