Not necessary, even if some asteroids could have occurred in the ocean, that doesn’t undermine Trent’s reasoning. It’s still the case that the Yucatan asteroid didn’t create enough dust, and that still constitutes a reason to doubt asteroids as the cause of dinosaur extinction.

Not necessary, because even if dinosaurs in the neighborhood of an asteroid impact, but outside the direct impact zone, still would have died, that doesn’t undermine Trent’s reasoning. Trent’s argument is that the Yucatan asteroid didn’t create enough dust to block out the Sun’s rays and cool the planet beyond the capacity of dinosaurs to survive. He could agree that the asteroid killed many dinosaurs around the impact; the point is that it didn’t create enough dust to kill dinosaurs worldwide.

Trent never argues that the dinosaur extinction had “many different causes.” He simply believes it wasn’t caused by asteroid impact.

Not necessary, because Trent can agree that the dust from the impact of the Yucatan asteroid had a cooling effect on the climate. His point is that there wasn’t enough dust to make the world cold ENOUGH to kill dinosaurs.

Necessary, because if this were not true — if MORE than one large asteroid struck Earth during the period when the dinosaurs were becoming extinct — then the fact that the Yucatan asteroid couldn’t have created enough dust by itself to kill the dinosaurs does not establish that the extinction wasn’t caused by asteroid impact. Perhaps the Yucatan asteroid plus another asteroid together could have created enough dust to kill the dinosaurs.