You do know that the fern spike-containing sediments are on top of the K-T clay, not on top of lava flows?
I don't know all places where a post-K-T fern spike has been found, there may be more than just Montana and NZ.
The Chicxulub impact was a global event, in terms of its consequences. Deccan-derived lava, however, covers a large part of India -- not of the world.
I don't know how local the fern spike is in America. However, it doesn't occur on top of lava. And, for the 12876398th time, volcanos are TOTALLY INCAPABLE of producing iridium and osmium with EXTRATERRESTRIAL isotope ratios, shocked quartz, glass spherules derived from ocean floor, and so on. Even if the crater were still unknown, the K-T boundary layer alone would be proof of an impact.
Overgrazing at the time of an impact? What a coincidence.
No, because such deer populations expand because of the absence of predators. This was obviously not the case at the K-T.
Probably, you say. You try to build a hypothesis on a speculation. Sorry to say that -- this is AFAIK considered pseudoscience.
Hey! Why didn't Triceratops spread around the worls WITH Tyrannosaurus? And what about the ankylosaurs?
How did Triceratops die out in the first place?
T. rex evolved when the regression began, AFAIK, or slightly earlier. According to your logic, it should have emigrated much earlier. The regression took 4 Ma, stretching the K-T boundary.
What is your evidence for a wetter climate at that time?
No problem at all for the impact theory, which rests on whole formations of evidence.
LOL. Hawai'i-type volcanism produces slight amount of iridium, but not osmium with extraterrestrial isotope ratios. And so on. I'm sure the archives are full of discussions about that.
"probably mostly conjectural"? I'd say "as sure as birds are dinosaurs" and recommend once more the book Night Comes to the Cretaceous.