Yes. The sunlight on Neptune is less than that on Earth, but brighter than the light of a full moon.
Yes and no. Neptune gets very little sunlight because it is so far from the sun. When light does reach Neptune, the red spectrum of the dim sunlight is absorbed my the methane atmosphere and the blue light spectrum is reflected off the methane, which gives the planet its blue colour.
You could see Neptune from Earth with a telescope if you knew exactly where to look.
No. Humans could not live on Neptune. The planet is extremely cold, its atmosphere is not breathable, and there is no solid surface to stand on. Although in the distant future we could see a space station in orbit around Neptune which would send probes, and ships into the atmosphere for scientific study.
You can see Neptune with a telescope.
Neptune is 30 times further from the Sun than Earth, so the fraction of sunlight it receives compared to the Earth is the square of that, so 1/900th as much. However, that is comparable with your living room in the evening with the lights turned on. It is still hundreds of times brighter than the full moon - enough to see in full colour. However, beneath the clouds there will be less light of course - it would very much be a twilit world.
Probably Mercury, since you would have to be looking in the direction of the Sun, the overwhelming glare of which makes it almost impossible to see anything else in that direction. Or, it could be Neptune. You can't see Neptune without a telescope.
Astronomers knew that Neptune existed before they could see it because they observed that the other planets orbited the sun in a way that could only be explained if they were being influenced by the gravity of another object of such mass. So the astronomers contemplated that there must be another planet somewhere that was changing the orbits of other planets. That planet is today called Neptune.
Probably Mercury, since you would have to be looking in the direction of the Sun, the overwhelming glare of which makes it almost impossible to see anything else in that direction. Or, it could be Neptune. You can't see Neptune without a telescope.
Yes. The tundra does get sunlight. Nothing could grow there otherwise.
Diamonds interact with sunlight the same way prisms interact with sunlight. You will see all the colours of the rainbow reflected by a diamond.
No. There is no water on Neptune. Neptune is a gas giant, which means it is made of gasses and has no solid surface. With no surface it is impossible for water to form. The blue is caused by methane gasses in Neptune's atmosphere reflecting back the blue light spectrum from the dim sunlight that reaches the planet, giving the appearance through our telescopes that Neptune is blue. But whether Neptune really is blue or whether it's entirely an illusion caused by light reflections is an open debate.
It's unlikely you will see many more than you could with the naked eye, however, depending on the magnification, it could be possible to see Neptune which is not visible with the naked eye.