Each of them coming up with similar stories around the same time isn't so much a coincidence as it speaks to what was happening to the climate at the end of the last ice age.

When I was young I dismissed the Noah's Arc story as just that, some religious myth not worth much thought. But taking a step back and looking at all the other cultures that have similar stories - Ancient Greeks, Byzantines, Mesopotamians, Myans, Chinese, Babylonians, Native Americans, Hindus, etc - there are far too many parallels for it to be pure coincidence.

I guess that depends on what you consider to be rapid. 20m/year is quite quick.

A glacial damn breaking would take a while to clear fron the region. We can see this today when hurricanes cause inland flooding that takes days to subside.