For the first part, no matter how bad a comet impact might be I doubt it would wipe out all life and humans in particular are hugely resourceful and adaptable to change and no matter what I think some pockets of humans would survive. It all sounds pretty destructive but comets are responsible for delivering the vast majority of water that we are lucky to have on this blue planet.
For the second part, if the comet impacted the earth, it would absolutely vapourise and nothing would be left to study except the crater it left! Maybe the atmospheric dust from the fallout of the impact could be studied. We’re far better trying to study comets in space where they are intact just like the Rosetta mission right now. Some of these comets are older than the earth. The earth has melted down and refined the primordial space dust that it formed from so we don’t get the whole story when we look at the earth’s rocks. Comets are composed of dust and even water ice from the early universe and by studying them we can find out a lot about the early history of the Milky Way. That’s why the Rosetta mission currently underway by the Euopean Space Agency is so important and should provide some a lot of really valuable data to help use solve some pieces of the puzzle of what the early Milky Way and Cosmos