Have you read the stories of August Dickmann, Horst Schmidt or Leopold Engleitner? Can you explain how it served their own selfish interests not to sign the declaration? They had every reason to believe not signing or not cooperating meant their death, and in Dickmann's case it did.
The facf that some signed the declaration doesn't detract from the fact many didn't.
The fact that many had complex motivations (peer pressure among JWs was another you didn't mention) doesn't detract from the many heroic acts of defiance against the Nazis.