I don’t know for the impedance question but, for the loose cable humming, you could try some Neutrik silentPLUG or Amphenol SwitchPlug. Thoses connectors ground the signal on disconnect.
Most instrument will probably work well enough on any input, it might matter more for a piezo pup. The idea of the high Z being 1M ohm is to simulate the tone of a preamp for a passive pup. I’ve seen it recommended that the input impedance should be at least 10x that of the instrument. If you’re running the DAI into a sim and not just using it as a straight DI, it’s probably not going to matter at all.
I keep the knob at around 11 o’clock with my pbass, i have no idea what that impedance is but my ears tell me that’s the right setting i’ve never used input 2 since i only play one bass/guitar at a time
I’ve got a Schecter Riot 4 with EMG 35JX-CS and EMG 35PX-CS and my interface is the Minifuse 2. I just use the same setup as my guitars which is Hi-Z button turned on and gain at 0.