First, definitely book this with an agent - they will look after you and make sure you get everything they can, whereas if you book through the cruise line, they look after them and not you.
As for a 'free room', your agent can explain how the Tour Credit works, but you need at least 8 cabins based on double occupancy to qualify.
As for the NCL Dawn, we were on her a few years ago and it's a nice ship - we enjoyed it. Definitely does not compare to the Oasis as they are completely different. In the Dawn, you're talking about a ship that is 11 years old, 92,250 gross tons, and 2224 passengers versus the Oasis, which is only 4 years old, 220,000 gross tons, and 5400 passengers. So needless to say the experience will also be completely different.
As for the dining, I tell my clients to allow an extra $100 or so because you'll want to try some of the specialty restaurants where, as with pretty much all ships, the food is much better than in the main dining room.
As I've said many times, food is very subjective and what one person loves another will hate. So you'll find people rave about the food in the main dining room on NCL, while you'll find others that find it only palatable. Personally, to us, it's okay - just a little less than RCCL. But that's just our opinion based on our 46 cruises on 11 different cruise lines.
While sailing out of Boston is convenient, it does require sailing in the Atlantic, which can often be a little rough. Just the nature of the beast, so to speak. But for seasickness I suggest trying ginger pills instead of the patch. Very cheap, no prescription required, easy to get, and no side affects. Just start taking them about 3 days before sailing.
As for what to do in Bermuda, sailing there the end of October is not going to be real warm. Think of the temperature as being roughly the same as the North Carolina/South Carolina border. And the end of October beginning of November has temperatures around the mid-70's, so swimming is not going to be something you'll be doing. But there are some nice places to shop and interesting places to see. And of course if you want to play golf, you're going to the right place.