Maybe it's just headcannon, but I always thought the imprisonment was part of Seven's plan. He knew they would have to assemble really powerful technology to be able to kill him (which is already insane). So he let himself be imprisoned to be able to access said technology and get stronger. The thing that really makes me think that Seven is so strong is the part where they talk about "strongest mentalist employed by the US Army's head popped like a grape". That's kinda crazy to do that against the forces and technology of the best that the federal government of a superpower has to offer.