Grammarians nowadays, however, tend to find it more helpful and accurate to reserve the term 'adjective' (unless otherwise specified) for the class of words that we would consider to be more 'archetypal' representatives of that group, e.g. noun-modifiers such as 'red, tall, happy' which share a common core of morphological and syntactic properties (e.g. the ability to be modified in their turn by an adverb such as 'very') that attributive nouns do not (so that we cannot say, for example *very baseball or *very chocolate).
Thus, to answer one of your questions, 'chocolate', by the lights of the contemporary definition outlined above, would fail to be classified as an adjective, no matter what its sentence position.
A copula can be roughly defined as a verb such as 'be' which can be directly followed by a noun that is not its object (as well as by a variety of other form-classes, as indicated in my previous post). A complement is any typically obligatory word or phrase that completes a phrase, clause or sentence.