Isn't only one of the mass extinctions (the dinosaur ending one) somewhat tightly linked to a comet impact? It seems as though it's finding a pattern that does no fit the data.
Also, I believe the dinosaur extinction was not just a comet impact, but a combination of an impact and a period of increased volcanic activity. Could the volcanic activity be triggered by comets?
I think the jury is still out, but it's an interesting field of study across many different fields (astronomy, biology, geology and probably more), and it has obvious implications for humanity's future on Earth, in space, and the chances of finding other life out there.
That's an interesting question. My completely unqualified opinion would be no, an comet couldn't trigger the massive and ongoing volcanic activity that a mass extinction requires. I'd like to hear from someone more informed though.
There is a local museum rigth next to the cliffs. A few years ago, I spoke to the leader there, a woman with a ph.d. in history, who more or less supplied the final death blow to any lingering respect I might have held for any kind of humanities: Her only take on Walter and Louis Alvarez and their world famous work in her own backyard was how of course they were so arrogant, they had raised hostility among scientists and everybody else the world over.