1. I'll have to say no, for no particular reason other than that Arlong was pretty friggin strong, and has lots of experience in battle. But on the other hand, water conducts electricity, so Nami would get him pretty good if she managed to hit him…
2. Probably. He could just spam some pop greens on the ground, like for example trampolia, and catapult him into the air and fisnish him off.
3. YES. Definately yes. Monster point ohko.
Let's do the other crewmates too!
I have to say that Sanji has become strong enough to beat Krieg pretty easily. I don't want to drag powerlevels into this, but he fought pretty equal with Gin, and Gin was stronger that krieg, right?
Robin would probably not be able to beat Crocodile. She wouldnt even be able to hit him, and she's not close enough to warlord level.
Brook could probably not beat Moriah. Luffy had to go into nightmare mode first, and then stack his gears in order to beat him.
Franky could....maybe beat Lucci. Franky has become pretty beast after the ts, especially with the Franky Shogun and his radical beam.
Zoro could definately beat Morgan (wich he alrady did) and Buggy.
And that's my two cents.