I wouldn't say Gravitron is "their" Magneto. They're most likely doing what "Arrow" has done and will use the series to push lesser known characters into the mainstream.
As for when Namor went to Universal...Fiege is the one who stated this, and I would guess sometime between "Iron Man 2" since Atlantis was shown as one of hot spots on Fury's monitor and now. I would assume the rights situation works the same way with Marvel Studios as it does with any other studio, they have to be actively developing a film with the character or else they become available. This kind of came out of nowhere.