This basic story has been pieced together twice previously, Donelly and Velikovsky, and Velikovsky at least provided an answer as to where the comet went or ended up.
Ya know? Maybe Velikovsky was close.
Let's look at Venus:
What is that thick atmosphere doing so close to the sun?
Why is the entire surface of Venus "young" (no cratering)?
and what's with that rilly rilly slow rotation?
The earth was hit by a Mars size rocky body that stripped off enough of our crust to form the moon, and allow us ocean basins and access to heavy metals.
Suppose Venus was struck by a large icy body, if the strike was a bit off center it could stop the planets rotation in it's tracks, shatter and roll over the crust, release vast clouds of volcanic gasses and dump enough cometary volatiles to form a thick bright atmosphere.
From the viewpoint of earth, Venus would go from being a medium bright star to being the most brilliant angry orange-red object in the sky.
If a lunar sized commet cut across our path on its way to an appointment with Venus, it would be the largest most dramatic display the human race has ever seen! As the coma grew it could easily appear 20 times bigger than the moon. And bright? Achingly bright, bigger than the sun, and nearly as bright. (or maybe even brighter, due to its size). The light would be cold, so cold, colder than Hillary's heart. and the tail would be scything towards us, to end in a sky aflame in meteors.
Clearly a single God swept by us and placed two flaming calls to war in our sky!
One as a relatively quick slap to get our attention, and a lasting burning flame to remind us.
None of that Velikovsky was right stuff either. Hey! I mean it. ~ carumba
And, of course, the HST watched the fragmented SL-9 disrupting Jupiter's atmosphere as it fell into the planet.
I think the Discovery Channel just had a Tunguska piece, and one of the 160+ theories was that it was a "dead" comet that exploded over Siberia.
So, maybe 12P/Pons-Brooks "died", and for all practical purposes, became a 'roid with a large orbit?