You are just justifying why the rule was broken.
When four pilots are boarded, the maximum allowable FDP for a single Sector will be eighteen
hours, regardless of considerations of acclimatisation or time of start of the FDP.
When is Commanders Discretion meant to be used? Did this chap go over because he felt he had to? I believe 18 hours is only to be exceeded in case of emergency. After the passenger was offloaded, the flight to JFK was strictly commercial not an emergency.
I did not realize the terminals/hotels were not heated in Canada.
If this chap broke the law, pressured or expected to break the rules, well something needs to be addressed. This chap may have to be the first to get his reprimand so the rest will not break the rules.
Like being configured at 1500', it is the limit, not the target. The limit is the limit. Ask the blokes that failed their commands because of it.