I don't get your point about Sanji
Arlong was probably the only person Nami would try to attack willingly, because he devastated her life. Still she begged luffy to beat Arlong's crew up for her sake. Like I said, Nami has always shown her back whenever an overwhelming opponent attack. Unlike Usopp, she doesn't have the sogeking persona, nor the expectations she has to live up to because of lies. Nami admits she's a coward, and she has proved always proved it. I never said Nami can't fight, she's a very efficient fighter against weaker opponents
And about Sanji, tell me what he would do if he sees somebody like Alvida trying to slit Luffy's throat?