That’s the type 1 with the big controller chip right?837-13551, usb seems to be fine, ground seems to be fine
Exactly this would indicate the loom is not fine as you measure yourself you’re losing over 2v on a line...The loom is fine, starts at the start button and splits out.
Buttons 5 and 6 on 2p are showing .5vdc when open while ever other button is showing 2.4 but jump up to 4.25vdc when either 2p button 5 or 6 is pressed