The 15 objects that I envisioned were: my dog, gumballs, raffel ticket machine, my moms car, a ferris wheel, rain, a windmill, basketball, my nephew, a bat (the animal), sparkler, a tornado, a thunderstorm, and a giant ball.
1) Actually I am about the same when visualizing people and objects its just when I havent seen a object before that it gets a little tough.
2) I am better at 3D objects interestingly enough. My imagination likes 3D when i try 2D..... nope just 3D xD.
3) its only when i envision nature do i actually have something of a background but other than that its floating in nothingness.
4) Hopefully its somewhere else because its kinda blank when i envision things.
5) In my field(animation) I would think it would be needed because say my boss wants me to design a character but he only gives me the bare necessity of what he is looking for. It is up to me to take what I a given give it a personality and a place. The mind is a great tool.