"Focused offense" prevent from "declaring" defenders, "The Wall" put night's watch character into play "as a defender".

My question is: does facused offense prevent popping characters with the wall to defend?

No, putting characters into play as defenders (The Wall, LoW Catelyn Stark, Horseback Archers, Guardian Wolf) or making characters participate in a challenge as defenders (both versions of Greatjon Umber) is not the same as "declaring" them defenders. However, the Naval mechanic is declaring characters as naval defenders, so it is prevented by Focused Offense.