Well, I have thought about the sand shell too. At first I was almost sure that was a shell, because of the pieces of sand skin covered nothing but a black spot. Thus, empty?
But, if it was empty, then the pieces of sand skin should have fallen in the empty shell instead of fallen of it.

About the chase of Kankuro after Sasori and Deidara, it's just impossible to set up a trap when you're the one that's chasing the other. First of all you should get ahead of the two, and then set up a trap and hoping they are going the right direction.
When you're the one that's fleeing (Deidara and Sasori) you can put a trap and mislead the one that follows you into the trap.

And it's a fact that Orochimaru is weaker then Itachi. Oro said it once to Kabuto, referring to the body he's always wanted (An Uchiha body). But since HE (Itachi + a blurry shot of Itachi) was too strong he went to obtain the body of Sasuke.

About Deidara's hand, I don't think they can re-attach his arm right away, since the Akatsuki only "meet" each other rarely (ep 134: The last time we gathered like this, was seven years ago when Oro left the group). And if they do, it's through that video thing in the cave. Don't think they were there in person.