1. you should have a spare spade plug for this on the rear wiring loom plug, E/14. The reason it has 2 wires is probably there are 2 components in the engien bay that connect to it, possibly the IAV or something
2. brown/white are almost always earths as well, usually for temp sensors. Ideally go back to the engine side and look for any brown/white wires, then use a multimeter to check for continuity. Then you'll know exactly what it is for
3. indeed, there will be a green/black spade plug coming off the dash wiring loom somewhere for it
5. most euro spec MK1/2 have this, all it does is bring the dipped beam on dimly with the lights. not required by law or MOT
however thinking on it again, I recon its actually an oil pressure switch wire, I remember now that I had the same behind my dash. You should find a matching spade plug coming off the dash loom, or to pin D/1 or D/21 on the fusebox
7. if in doubt, leave it unplugged! This one looks like its coming from the fusebox plug in the same picture?
8. I would plug it in yeah, then check everything works