- To the argument whether insertion of foreign DNA sequences is mutation or not: A mutation is defined as an inheritable modification to the DNA. Thus processes involving inserting foreign DNA are also mutations. However this is just a matter of wording. The fact is that this process, whatever called, can add new sequences to a DNA that are then inherited, and thus can add information to the gene pool.

- As to the comet question: There are short-periodic, long-periodic, and non-periodic comets. Non-periodic comets are those that only appear once and then disappear into the depths of space. Comets begin to evaporate when they are close to the sun, thus their life span can be rather accurately calculated from their size, perihel and orbit period. I do not know offhand of a comet of only 10,000 years life span, but an example for a short period comet is the Halley comet with an estimated life span of 40,000 years. Long period comets are much longer lived, like millions of years.

When calculating the paths of comets, you can trace their path back to the place of their origin. Long period comets mostly come from a spherical region about 50,000 AE from the sun, named Oort cloud. Short period comets originate from within our solar system, either from the Kuiper Belt or the Asteroid Belt.

Comets do not indicate a young age of the universe. But indeed there were a lot more comets in the early time of the solar system. The geologic record of the earth shows that it was heavily bombarded by comets and asteroids until about 4 billion years ago.