Yes, sometimes there are discounted rates depending on how full the ship is. We did the first Mediterranean cruise this season and the ship was at 40% capacity. So I could have rebooked a verandah for what I paid for oceanview. If you go on
DCL's website, you can get a feel for the discounted cruises.
We did the standard Alaska itinerary out of Vancouver (Skagway, Ketchikan, Juneau and Endicott Arm). It was amazing--drove the Klondike Highway, flew in a floatplane, went whale watching.
Yes, if you go on a short cruise, you can book a placeholder for $250 and then you can use that placeholder to get 10% off your next sailing within two years (there are blackout dates). Also, you can't apply the discount if it is a discounted sailing (I couldn't move from Ocean View to Verandah on the Mediterranean because I'd used an OBB. I could have rebooked but would have lost my OBB discount).