NCL is just telling you to stay on top of requirements. it has always been the passengers responsibility regarding visas and they are just letting you know that you need to be alert for any special health requirements so that you don't show up for boarding and get denied. Letter is nothing to be concerned about.
Never understood the reason to be the one to contact NCL about a M&G. You don’t get anything for doing it other than getting to pat yourself on the back. Definitely a first world issue getting so upset.
Those times are woefully incorrect. You can generally expect to be able t get off the ship within 15 minutes of arrival time. All that other stuff is taken into account when setting arrival time.