Kim, I can't help with the cabin selection question but we have others on the board who are Navigator cabin pros! :)

For question two, it depends on your itinerary. Some ports have lovely dock areas,some don't and some ports, you will tender in. I don't really worry what side of the ship I am on for the Caribbean. Do you know your itinerary yet?

For question #3, if your travel agent does further discounting, I would assume that they would advertise that up front. If not, it wouldn't hurt to ask.

As for the last question, you would have to ask your agent that as well. If you pay with a certain American Express card, you will receive an on board credit and some travel agents also offer credits.