depending on the type of vehicle you own yes it can harm your transmission. i would always use D due to better gas mileage, and smoother driving.
They "reset" while driving at highway speeds.They "reset" while driving at highway speeds.
If by this you mean only "hot heat" when drive at highway speeds likely thermostat bad and/or almost stuck and takes long time warm up from high engine RPM during highway driving.
Possibly your speed sensor. My speedometer would bounce around & then soon after it felt like my transmission was slipping. Turns out the speed sensor controls the upshift in your transmission.
The distance behind the car you are following
That will depend on how fast you are driving. About 1 hr 45 minutes at highway speeds.
Highway driving tends to be consistently moving forward, whereas city driving is a lot more stop-and-go. Higher speeds are reached on the highway. Highway driving tends to be straight line and long curves, whereas city driving has a lot more turns.
Yes, it helps when driving at highway speeds.
Yes, if you had to make a sudden move at highway speeds you may lose control.
You should use d4 because that means it will use all four gears in your automatic transmission. D3 will only go from 1-3 which is fine if your not going highway speeds.
At highway speeds yes, in city driving no.
2-3 rpms
Cars are rated as to how efficient they are. The government gives them two ratings; Highway mileage and City mileage. City mileage is determined by driving the car at slower speeds and stopping and starting many times. This consumes a lot of fuel during the times the car is not moving or having to accelerate. Highway mileage is determined my driving the car long distances at highway speeds, usually 55 mph or more.