I don't really get what you mean. The first live specimen was found as bycatch on a trawler ship, but we still have no fossils of any deep-sea lobe-finned fish. And as for the second question, the K-Pg line is about a centimetre thick, it would be pretty damn hard to find a dinosaur there. We have them beneath it, and we don't have any non-avian dinosaurs above it.
That is my point if they died to the asteroid strike. How come there aren't any in that layer. They would have died then that centimeter of dust would settle around them. Thus there should be foaaels in the layer.
As for the fish. The men who knew what it as saw it in a fish market. The fishermen had been catching them for years. I think they were being sold for dog food. They don't taste good to us.