The first 2 pics appear to be a pistol shrimp, could be a mantis shrimp but I wouldnt be able to tell without a better pic as Im def. no pro on them.
The last 2 are indeed exoskeletons. There is no way that thing is moving around, unless its catching flow. In fact the last pic, you can clearly see where it's thorax and abdomen snapped in half and are just barely hanging together. Another thing to note is, theres no guts visible, even ghost shrimp you can see their innards.
-Justin