1) I agree, early or late, only problem can be shadows. If you already picked a location, be sure to check out where the shadows are in the morning and evening.
2) anything goes, just avoid it beeing low behind the car
with the polarizer you won't have any other major problem
3) No fixed rules... you just have to test. Actually since you have some time, go out and test everything on cars on the road, friends' cars... would be a pity so screw up a photoshoot
4) No, only use it when it's dark and and can't have enough shutter speed at the given F seetting.
Yeh, don't be afraid to move the car. Also, pick at least two different locations if possible to have some variety. Don't forget the way the wheels are turned
Oh and of course, NEVER use the flash. Only when you have big balls professional ones it can be worth, if not, it will just be crap.