Mike, YOU introduced that commandment to the discussion.
According to the verses I referenced, God Himself approved of the killing and rewarded the killer. Call it an "exception" if you must but at least recognise that according to the Bible it is God who made that exception and who strongly approved of it. Every time you say that I am just arguing from an event in the OT you ignore that and misrepresent my point.
Have I considered the "whole teaching" ? well since the teaching we are talking about is an OT teaching any modification in the NT is irrelevant, and if the verse you refer to does NOT modify the teaching then the answer is of course that I have considered the whole teaching - anything that says that Phineas' action is wrong must contradict the verses which state that God found Phineas' actiosn to be good and worthy of reward. And that leaves us with no consistent teaching.