Apparently. I say that because the bird fossil record in the neighborhood of the K/T boundary isn’t very good, so it’s hard to tell what happened.
All modern birds are post-K/T species.
No. Most genera are probably Miocene or later.
We don’t know of any non-flying ratites that made it through the K/T boundary or that lived before the boundary. The data suggest that loss of flight happened later.
No. We can suggest a minimum based on time-calibration of trees. If I recall, that minimum is around 7. But we don’t really know how diverse birds were before or immediately after the extinction.
Incidentally, any attempt to match Genesis 1 to reality is doomed to ignominious failure.