You make it really hard not to be mean, Superbear 22… Usopp a legit pirate captain before joining? Nami having haki? Please tell me you aren't serious...With Nami it's a gag. Part of the gag is that only her in a rage is capable of harming Luffy. The fact that even Sanji doesn't when there's a gag with him angry at Luffy is just part of how Nami's gag works.
Edit: And no, I never wondered how she could harm Luffy, Oda practically said it was a gag when brought up in SBS. And the same gag concept applies to Luffy hitting Smoker/Ace, but that was a one-time thing needed for that scene to play out that way