Does a fly, flying around inside a car (windows closed, hermetically sealed if you like, make the car heavier???
Hmm. What happens if you hold a weight - without resting it on any interior part of the car - through an open window of a car standing on scales, that's not going to make the car weigh more. Only difference with the fly would be the closed windows.
Incorrect. If you hold a weight outside of the car through an open window, the center of gravity is still inside of the car. By holding the weight up, you are weighing yourself down.