I don't know what his being a teacher of Hebrew would have to do with his abilities in koine, but in any event...
... in the Greek whenever a person was being addressed as "My Lord" it was always (Mou Kurie) and not (Mou Kurious).
In the case of John 20:28 the o kurios mou may not be intended as a direct address (and, therefore, would not be in the vocative).
However, the use of the nominative in exclamations can be used as "'a sort of interjectional nominative' (Robertson, p.461) and is a device for emphasis." Also, "[m]any grammarians call attention to the nominative 'in place of' or 'as' a vocative." (Vaughan and Gideon Greek Grammar of the New Testament p.25.)
In Matthew 16:23 and Mark 8:33 Jesus does address Peter as "satan" ("adversary") or "Satan" (presumably metaphorically); the name is in the vocative ( satana ).