I don't recall that about Agatha, but I'm not entirely familiar with her history.
REH = Robert E. Howard. Marvel tied in Namor's Atlantis with Howard's mythos -- Conan, but moreso Kull of Atlantis. The Serpent Crown is related directly to that ancient age and Howard's Set. Some of that was related in the old Tales of Atlantis backup stories in the Sub-Mariner's silver / bronze age book.
The magical stuff has been scattered throughout Namor's appearances from the silver age onwards.
The Morgan Le Fay stuff was added on, along with the Inhumans Attilan, in the last issues of Namor's 90s book, the Atlantis Rising issues, and some other tie-ins.
I don't recall that about Maelstorm or Agatha, but again, I'm not that familiar with them. Now Ultimate Salem Seven had some connection to Ultimate Namor's Atlantis, IIRC. They were enemies of the Atlanteans -- but then again, so was Namor, sort of.
I think some writers tried to connect the Deviants to the Atlanteans, and definitely to the Lemurians, but I think that was another retcon / add on that disregarded stuff that came before.