Neanderthals are probably the most recent human species other than sapiens but denisovans are a possibility.
Stories of great floods aren’t exactly unknown - there is also speculation that the one found in the Bible is a record of an end of the ice age phenomenon. But tying them to a specific event is difficult. Note that multiple ‘historical’ explanations have been advanced for the same myths.
Heh, no need to travel to the Okneys to find mysterious human-built mounds. Those things were dead common, in Neolithic burials - the more probable inspiration for stories of this sort.