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If you are looking for directions to a port, check out the section "Getting ready for your cruise" on NCL's website. It has the addresses for each port under "Getting to the pier"

According to the cruise map, the ship sails around the north coast of the Big Island. So, NO, you won't see the lava flowing into the sea at night, like we did in 2009. At that time most of the cruises sailed around the south coast from Hilo to Kona.

Yes!

Usually they start the check-in process about 5 hours ahead of the sail time and start boarding 4 hours ahead. You have to check in at least 2 hours ahead of sail time. If you are at the port around 4 PM you should be fine.
If your flight gets delayed, make sure you contact NCL and inform them, so that they are aware that you may get delayed.

The weather is in the 80's year around. The winters are slightly more pleasant. Avoid the holiday season if you can.

Most car rental companies allow you to rent from one location and return it to another location as long as it is in the same city. You will have to check with your car rental company if they charge a "vehicle recovery" fee.

Unfortunately there is no way you will be able to make a 8 AM flight. The ship docks at around 7 AM. Once the ship is cleared by the port authorities, the first ones to disembark are the VIP guests and guests carrying their own luggage.
Even with early disembarkment you still will be at the curb around 8 AM. So a 8 AM or 9 AM flight is out of the question.
To be safe, I would say book a flight after 12 PM. If the flights are too expensive, you may want to consider booking into a hotel for one night. It may turn out to be cheaper that way.

The ship docks at Honolulu Harbor, Aloha Tower, downtown Honolulu. It takes 15-20 minutes by taxi from the Hilton Hawaiian Village, which fortunately is at the west end of Waikiki, nearest downtown. You come out of the resort onto Ala Moana Blvd, heading west, straight to downtown.

Probably not. Per the scheduled arrival time on the itinerary, the ship anchors about 7:00-7:30. Then you have to tender ashore to the pier. It's not far to the pier, but it still takes time to board the tender and make the 10-15 minute shuttle ashore.

Yes, both are provided, a dispenser is located on the shower wall.