1. Yes, on the ship you start an onboard credit card account and you either make a cash deposit or have a credit card signed up.. Then whenever you buy something with your card it will just be charged to the credit card. This is how I understood it, when I was cruising, but rules could have changed.
2. Most every cruise terminal has a place to park, and if you are concerned about fees and accommodations, you could just call the port, and see what the fee per day is and see if they have parking. Customer Service would probably help you the most in this area.
3. It's just kind of hit and miss. I mean the porters once you get to the port, find a porter to take your luggage, and make sure you give it to the right cruise line porters, so you luggage makes it on the right ship, and make sure you have all your buggage and luggage tags, with cruise ship, stateroom number, and name and address on your bags. Then they choose random bags and check them, but I am pretty sure all the bags either so through scanners or get wiped down, kind of like on airlines, but then again I dont know, just that you give them to the porter and go check in..
4. The privacy is whatever you want. I mean if you want to be alone in your stateroom so be it. You don't have to participate in any activities, or events that you dont want too, except for the life boat drill, which is mandatory. It's really fun, not.
Also alot of people forget this, bring alot of $1, $5, and $10, for tipping money, on shore excursions, and just tips for bus drivers, tour guides, taxi drivers, porters, etc. You never can have enough of those...