It continues the storyline of a few characters from the original Watchmen, but takes place 34 years later with a completely different premise and setting. Therefore it is a loosely based sequel, and reading the original comic series/watching the film adaptation will help your understanding but is not necessary.

Loosely based on the comics, the original author has taken a stance against the tv series (as he does with all adaptations of his work) but whether it's canon is a matter for DC who owns the material. DC is a bit weird however since the DC Comics "Doomsday Clock" seems to be in direct contradiction to the TV show.

I would characterize it as entirely canon to the original comic series; completely unrelated to the current "Doomsday Clock" series, which is a modern comic series that goes in a very different direction; and not entirely canon with the movie, since the movie departed from the original comics in a significant way at the end.

It most certainly is not! It is much more complex than that. It tackles real world societal issues, aswell.

It shot specifically for the show

Showrunner Damon Lindelof had stated in an internet news article that he wouldn't write a second season unless he has a good story that will complement the first season or succeed it in terms of story. Therefore it could be considered a miniseries.