According to his own story, once he regained his memories, what prompts his assault on humanity is the fact he believes Atlantis had been destroyed by it. I think you can easily establish a narrative in which Krang used the Serpent Crown to make Namor amnesiac decades ago after a big battle, and he slowly is able to piece it together over the course of the movie. Perhaps the Serpent Crown becomes the central focus of a future Avengers movie, with Namor heavily involved. This could establish Namor as an uneasy ally as well as a sympathetic villain.
It's just a promotion for the Kings line, which Black Bolt was a part of too. Later expanded to include more figures.
Hard to say which arc since it's been so long. The last great run was probably his mini The Deep though. Very different take.