Because the utility customer is providing energy to the grid. Usually because of net metering of renewable systems.
Because the meter is connected backwards
You do not remove the electric meter from a house. Ever. You could be electrocuted or have an explosion in front of your face, putting molten metal through you. You must call the power company and they will send a properly trained and properly safety-equipped technician to remove the meter from the house. Even the fire department is no longer authorized to "pull a meter". They would rather have your house burn down than risk the life of a firefighter making an attempt to pull the meter, even with firefighting protective equipment. Do not attempt to remove an electric meter from a house.
You would not be the first person to have someone else tap into your electric power. If you are in a detached house, it is fairly easy to find out. Turn off everything in the house. Leave the circuit breakers on. Have someone stand outside by the meter. See if the meter still runs. (You may need to turn off the breaker to the hot water.) If the meter is still running when nothing in the house is turned on, you can call the power company to come out and check.
In BC, Canada it's the same way as a house, and I guess would be the same anywhere. - The electric company places a meter on a pole outside the house and the homeowner gets a qualified electrician to connect wiring from meter to power panel in home.
A voltmeter or a multi-meter.
Usually you just have to replace the electric panel and maybe the electric meter and entrance cable. It would be unusual, but the power company may want to replace the cable from the local transformer to your house.
In some cases yes, but it depends on the motor's application (use). For example, the electric motor in a batery-operated toy car will turn backwards if you reverse the polarity. Other more complex motors won't work at all, or can be damaged if you reverse the polarity. it is an AC motor that runs the Wattmeter, it is all about the direction of current flow to make the meter run backwards you would need to swap the incoming active and the incoming neutral with the outgoing active and the outgoing neutral, thereby reversing the meter. reversing the Active and Neutral does nothing because they already are swapping between themselves at 50 times a second or 60 depending on your location. Also if you put more current into your supply than is being draw by it then the meter will also spin backwards. eg: overdriving it with a generator.
If your circuit breakers are in the garage, they would be in a electrical panel (a metal box mounted on a (usually) outside wall, with the electric meter on the outside. The circuit breakers can be either just below the meter on the outside on opposite it inside the garage. It would usually be very close to where the electric connection to the house is. The electrical panel and breakers are not necessarily in the garage.
If there is an electric short, the resistance of the circuit would be zero. So you can use an ohm meter to test it.
You measure a house fly in millimeters
In most houses you would.
It's possible, but everything would have to be installed backwards, windows barred etc.