Yes.... item 3 is a ground connection to the vehicle body. Most wires labelled with N (Nero = black) are ground wires.
Yes.... the GR (Giallo/Rosso) wire provides a +12 volt supply from fuse 5 in fuse/relay box 96, to rheostat 101 and the headlamp adjustment motors.
This old eBay listing: https://www.ebay.co.uk/itm/Genuine-...ment-Dashboard-Switch-735268798-/274106972575 shows it as part number 735268798. The Fiat parts catalogue shows 735268798 and 720953080 but both are no longer available from Fiat. I don't know what difference there is with the 2 part numbers. It could be one is brown and the other black plastic.