Apparent changes in nebulosity intensity are almost always a result of image processing. It is possible that moving blips may be asteroids. Asteroid orbits may be inclined as much as 30 degrees to the ecliptic. M100 is located less than 20 degrees north of the ecliptic. Some pixels in the detector have electronic problems, and may be unusually bright.
Some bad pixels are permanently bad, some are intermittent.
The professional astronomers did not blink images to find Cepheid variables. They used sophisticated computer programs to hunt for them. The computer programs could distinguish subtleties the eye cannot.