Most discounts listed in the cruise planner are smoke and mirrors. You just have to look at the current cost at any given time on the planner and decide if it is a good price, knowing you can cancel and rebook if the price drops.
As for the two-day discount, the only way to know if that is a better deal is to find out the price on board and subtract two days worth. I suppose you can get an idea of what that price might be by looking for a "live from" thread on the same ship when you get closer to the day of your cruise, but even that is not a guarantee, especially with a different itinerary.