We did the POA last year. We spent a week on Oahu, then the week-long cruise, and ended up with a week in Maui.
I tried posting a review on here, but it's too long, so I could do it. If you're interested in reading the review, which will answer alot of questions and provide some great information, send me a PM and I'll be happy to forward the link. Unfortunately, forum policies do not allow me to put the link in this post.
But definitely book through an agent as they can be invaluable when helping you with all the details and questions you have.
As for how to see Hawaii, the 10-12 night one-way cruises either to or from the West Coast are a better deal overall when looking at the number of nights. I just priced one for a client and an 11-night cruise on the Celebrity Solstice was basically the same price as the 7-night on the POA. And while the itineraries are slightly different and the Solstice spends 5 nights at sea, it is MUCH better than the POA.
If you think of the POA as just a way to see all the islands, you won't be disappointed. We enjoyed the ship and especially the crew, but the food left alot to be desired - not very good at all. The ports are awesome and I definitely suggest renting a car in each port versus shore excursions. They are cheaper plus you can do what you want, when you want, and go where you want. We are so glad we rented cars.
And yes, NCL will lower your rate if the prices go down provided final payment has not been made.
As for bringing wine onboard, NCL charges a $15 corkage fee for any 750ml bottle you bring on board, even if you're going to drink it in your cabin. Check it out here; Frequently Asked Questions
Let me know if you have any questions.