The item bonus is just what is listed in the armor table as the AC value. The item level of the armor does not influence the AC you get; it is used to determine what level a character needs to be to craft it and how high the DC for the corresponding Crafting check is.
Thrown hatchets generally do not get the cleave on crit since the effect specifies that the second target needs to be both adjacent to your target and within reach. Note that the term "reach" always refers to melee attacks; for ranged attacks, the term "range" is used instead. There are corner cases, such as throwing a hatchet at a target 10ft away with one other enemy between you and the target; in this case, as written, the critical specialization would apply. You can flavor this however you want; my interpretation would be that you graze the second target while throwing the hatchet at the primary target.