your scrodom gets kissed
nothing happens
Halley's Comet was observed and recorded many times in many places around the world. Sir Edmond Halley demonstrated that the repeated sightings were of the same object, the comet that now bears his name.
Twice; 1910 and 1986.
350 times
Once or twice a day but make it small
Depends on the comet.
Yes. The surface of Mars is littered with craters from such impacts. In fact, every planet in the solar system has been hit by a comet... not just once, but many, many times!
The gasses liberated in the comet's tail are very diffuse. So it takes many trips of a comet about the sun to bleed off a lot of mass, unless the comet passes sufficiently close to the sun (or collides with it or with something else). Eventually the comet would become a much smaller, rocky body, sort of like an asteroid. All the water vapor, methane, and ammonia will have bled off.
As many times as it happens.
nothing
People of the Comet has 131 pages.
In the Days of the Comet has 378 pages.