Not for this poster in the UK, but the deadline is the final payment date, not the day the payment is made. If you pay off a cruise in March but the final payment deadline is in April, you can still get the adjustment.
11:30 is your port check-in time. You may or may not board precisely at 11:30. If the ship is not ready for passengers, you'll wait in the terminal. Once the ship is ready for boarding, they'll let groups on and you'll board after those who arrived before you.
Well, after he missed the turn for terminal 1 (or is it A?), we drove all the way back around to get there. I got off at the first stop, but I am pretty sure people remained on for a second stop.
And yes, $10 is a great deal.
Luggage for transfers should have certain numbers and will be grouped in the terminal for pickup.
I'd arrange for a relatively early disembarkation group and just take the $10 shuttle. People will direct you to it when you exit the terminal with your luggage. Smaller vehicles, less time to fill up, less luggage to sort at the airport.
See also the Florida Departures board.
Re: parking
Passengers from the prior cruise have to depart by around 9:30, and the terminal won't open until at least 10:30-ish, so there's no point in arriving while people are still leaving.
Reflection in August/Sept. had pop-top water cans (can't be closed once opened), as did the Constellation's TA. Perhaps only European cruises have pop-top water, but it's a thing.