Setzer, your theory rules! (I may have to borrow that for a sig, if you'll let me).
About Nami and Zoro :
The key thing to remember in the Arlong arc scene mentioned above is that Zoro had previously teased Nami because she was not able to take someone's life. She had the opportunity to kill Zoro twice at Arlong park, but both times, she spared his life instead of taking it.
So when Yosaku (an old friend of Zoro's) said he'd witnessed Nami killing Usopp, and when Nami herself confirmed the rumor, Zoro probably felt as if Nami had only killed Usopp to prove to him (Zoro) that she was capable of killing someone! (I doubt Zoro would have teased her about her inability to take a life if he felt she would race off and kill someone just to prove him wrong).
That's why I think Zoro fought Nami : because to him, the evidence pointed to her having killed Usopp.
Now, for Sanji and Luffy :
Ok, think back to what Luffy and Sanji knew about Nami then. They knew that Nami had stolen the ship, and that she was headed for Arlong Park. They knew that while she was with them at Baratie she was friendly, cheerful and a bit flirty. They knew nothing about her relationship to Arlong ; nothing about her contradictory behavior towards Zoro while he was a captive of Arlong ; nothing about the reasons why she'd been accumulating treasure. So when they see this charming young lady (who inadvertantly stole a ship!) and hear stories of her being a cold-blooded murderer – as told to them by a guy they've only recently met and may or may not trust yet -- ... which impression do you suppose will prevail?
Add Luffy's set-in-stone insistance that Nami be his navigator, as well as Sanji's inability to doubt the motives of any woman, any you can see why they'd refuse to believe that Nami is a murderer!
And even though Nami is proven not to be a murderer eventually, she'd given Zoro such mixed messages up until that point, that it took a long time for him to learn to trust her. Something like three or four arcs ...