Yes you usually can depart the ship at an earlier port on a transatlantic cruise, BUT it must be arranged in advance with the cruise line prior to sailing. Most, if not all, have procedures in place for setting this up. There are times where this isn't possible (the most common example would be if it would turn a round trip cruise from the US into a one way cruise between two US ports) but that isn't an issue for your particular itinerary.
You also must have a plan in place about what you will do if the ship has to skip Halifax. On my first transatlantic cruise, we had to skip a stop in Canada due to a massive arctic storm. We actually ended up getting more time in a different Canadian port, but if anyone had planned to disembark at that first Canadian port they would've been quite unhappy.