why is "everyone" in #5 translated to some and not most? Wouldn't everyone seem like most?

For #3, I get the explanation and that was how I did it. However, since "no" is in the negate necessary, doesn't that mean "no myths would have been written down" have to be in the necessary side? I know we were following the unless condition, but so does that mean we can just ignore the negate necessary condition for "no?"