1 ~~ In this sentence, "open" is an object complement. It's an adjective modifying the direct object "it."
Scroll down to the COMPLEMENTS section and read the second example given for Object Complements. This is the same construction.
2-7 ~~ At first, I would have told you they are just two words for the same thing, but I have learned they are not!
So ... I'd use "cacao" when referring to the plant from which the beans come, and I'd use "cocoa" when referring to the beans and the chocolate powder or chocolate drink made from those beans..