case for predicate adjectives
Throughout the course it seems that predicate adjectives after был, etc are in the nominative case. If I understood my college Russian classes (and grammar reference book therefrom), they are supposed to be in instrumental (with the exception of nationalities). Is this reflecting a change ongoing in the language (that my textbook of 5 years ago was already presumably well behind the times on)?
In modern Russian predicate adjectives are only used as a predicate and only have the Nominative case. Exceptions to this rule can be found in some old set expressions (e.g., на босу ногу "barefooted").
A long time ago, in Old East Slavic (a.k.a. древнерусский язык) all adjectives were "short" and declined pretty much like nouns. A demostrative pronoun could be attached to that bare adjective to express that a specific item is in mind. The forms were и (masculine), я (feminine), е (neuter).
Later, this demostrative pronoun merged into the adjective, which is why third person pronouns and adjectives still have very similar declension pattern. "Bare" form, on the other hand, stopped being used as a modifier and was only kept in predicates. Nowadays the full form is perceived as the central form of an adjective (besides, many adjective do not have short forms), and short forms have no cases other than the Nominative.
If you meant full form adjectives (and nouns) used in predicates—yes, there is variation in use. The traditional view is that the Nominative is used for things that are the essense (the subject IS this thing). The Instrumental is used for things that are "roles" or properties (subject becomes this thing and maybe eventually stops being this thing). In practice, the borders are often blurry. For example, imagine an old woman talking about the time when she was a little girl. She can say "Когда я была маленькая..." or "Когда я была маленькой..." — and both variants are found in real life.