1) You can left the lens attached to the body, depending on what type of camera bag you have. If backpack type, it's better to take it out. If those square type, I believe your zoom lens can't fit in, so most of the time you'd have your kit lens attached, like mine.

2) Shouldn't be a problem.

3) Leave it in your car's cabin and let it enjoy the aircond. But do take note of this. Once you took the bag out of the car, into the hot sun/temperature much warmer than your car, DO NOT take out the camera immediately. Let the temperature increase on the camera first, else moist would be formed due to condensation. (Actually, this should've been stated in your user manual)

4) Forget about the xx thingy.

5) Basic calculation is, take the longest focal length divide by the shortest focal length. Eg: 300mm / 75mm = 4x. But the 4x only applies to this lens, as in the magnification from shortest focal length to the longest. This is my understanding, correct me if I'm wrong.

Enjoy your new toy!