Looking for Answers about Nikon Inc. 1 NIKKOR 18.5mm f/1.8?

The lenses for Nikon 1 cameras have a different lens mount than the D90. I have not seen an adapter to use Nikon 1 lenses on d90 cameras. There is an adapter to go the other direction, i.e., to use your AF-S lenses from the d90 on Nikon 1 cameras. This is intriguing to try since a 200mm AF-S lens would become a 540mm lens equivalent on a Nikon 1 camera.

You can use any lens for portraits, this is just a longer focal length so you wouldn't need to be as close to the subject. As far as the low light performance, this lens has a higher aperture (f/1.8 compared to f/2.8) meaning that it allows more than double the light to hit the sensor. This means lower shutter speeds and/or lower ISO resulting in less chance for motion blur and/or less noise, respectively.

The 18.5 mm is a lens for the nikon 1 series of cameras, it will not fit your camera. You need a AF-S or AF-S DX in order to autfocus, bothe the 35mm 1.8 and the 50mm 1.8 will give you excdellant bokeh (blur) and will be good for low light. The 35mm will give yuou the widest viwing angle of the two but is not a wide angle lens. If you don't mind spending the money the AF-S DX Zoom-Nikkor 17-55mm f/2.8G IF-ED would be an excellant choice for all your needs.

No, this lens will not work with the D3100. This lens is made for use with the Nikon 1 cameras, which have a different bayonet mount than the 'standard' Nikon cameras and use a smaller image sensor.
I do now know of any adapter that would allow this lens to be used with a D3100, but adapters do tend to reduce photo quality slightly.

For a D3100 you might want to take a look at the 50mm f1.8, which would be the equivalent to this lens for the DX sensor format used by the D3100.

No

It will be available from the beginning of November

No manual focus on the lens, must be done from camera.

We currently do not have the lens available at the nikon store.The lens is currently in stock at Authorized Nikon dealers; Amazon, B&H photos, and Adorama.

Image noise is created my a camera sensor or film, not the lens.
http://www.nikonusa.com/en/Learn-And-Explore/Article/g9mqnyb1/Understanding-ISO.html