Maybe. This lens will allow you to shoot in lower light conditions than the 18-55mm lens that came with your D3100. This might include shooting indoors without flash and in museums and at shows with fast moving people, assuming you can get close enough. I own one and like it but I also have other more general purpose lenses.

I have a feeling that if you have to ask, this lens might not be the right choice for you. A wideangle lens that goes from about 10-20mm will open up a whole new world of photography not available to you now (good for some travel shots, but stitching photos together might also work) and a lens with more reach such as 18-105mm or 18-200mm might have a more immediate benefit. Portrait photos typically use a lens somewhat longer than 55 mm (maybe 85mm on your camera). One thing that is especially nice about the 35 mm lens is its compactness. With this lens I can put my whole camera into a vestpocket pouch and shoot images in an art museum that does not allow flash. I have framed copies of several masterpieces I've taken this way.

Because many people like a little more reach, the kit lenses of cameras that cost a little more come with the 18-105 mm lens. Also there is a 55-200 mm lens that can provide extra reach without duplicating what you already have.

Yes, the 35mm f1.8 has its own AF motor, so the D40 can be used with no issues.

That will work perfectly as the 35mm lens has its own focusing motor. The sb-400, especially when bounced, will give plenty of wide light to capture with the viewing angle of the 35mm lens! Cheers!

yes! D7000 is a camera in DX format and this lense too.

No. With this lens you need to "zoom with your feet", but the advantages of this sort of lens are it's size, weight, sharpness and and the ability of it's wide maximum aperture to gather light. It's ideal for general photography, street shooting, and low light photography without flash.

Yes, this lens will auto focus with D90 and with more simple DSRL - D50, D40 (which does not have internal AF motor inside the body).
D90 works with:
-all AF NIKKOR lenses D-/G-type (excluding IX NIKKOR lenses) with all functions possible (including 3D-Color Matrix Metering II mode);
-all DX AF NIKKOR lenses (all functions possible);
-all AF NIKKOR lenses other than D-/G-type (excluding old lenses for F3AF) will also have all functions, except 3D-Color Matrix Metering II mode;
-all old AF Nikkor without D-type will can not work in 3D Color Matrix Metering and 3D Multi-Sensor Balanced Fill-Flash mode.
- non AF lenses AI-P NIKKOR will not work in autofocus mode, 3D Color Matrix Metering II and 3D Multi-Sensor Balanced Fill-Flash mode.