I played both Multiplayer and Co-Op. No storyline. Sorry, can't really give any storyline info.
AI was pretty good. In Co-Op my buddy and I were able to pin down the covanent while the other flanked them. So while the AI was good enough to get them to seek cover, they didn't react well to a combined assault. Again, this is an old build beta so may not be fully polished.
Dual Wield was pretty good. Not just limited to pistols!
I personally didn't use the dual wield much as it wouldn't allow you to chuck grenades during dual wield. Also, you could mix and match guns for dual.
