Throbert,
Your college teachers of Russian taught you well. Indeed, DEVOICING (or: "devocalization") of voiced Russian consonants /б/, /в/, /г/, д/, /ж/, /з/ in the end of Russian words is a norm. You provided several good examples to illustrate this devoicing.
However, the tendency to pronounce the inflexions "-ого" as /ава/ and "-его" as /ива/ is a different rule, because 1) there is no devoicing of "-г-" to /к/, and 2) this rule applies not just to any word, but to adjectives, pronouns and ordinal numerals, as was shown earlier.