Welcome!
I'll work on some of your questions....
The port area opens at 10 to incoming traffic, the terminal opens for check in about 15 minutes later. This depends on the disembarkation from the previous cruise going smoothly. Boarding the ship starts about 11:45, and you MUST be on board by 3:45. The standard taxi rate to the port is about $150 although I've known of some cabs going the other direction to negotiate for a fixed fare of $80. You are right that a towncar service is about $100-$120. A rental car about $40-50 for a full size vehicle. Car seats are $12 each from Budget. So...you're looking at $140 for
DCL transfers, $120 for a towncar service, or about $75+ tolls and gas for a one way rental.
The drive from MCO to the port is about 45 minutes. Of course you need to factor in getting luggage, getting rental car (or meeting car service driver), dropping off car, etc. If you do rent a car, sign up for the "frequent renter" program for the company--it will save you a lot of time!
You are correct that a child must be fully potty trained to participate in the Oceaneer's Club. This means that they can handle the going as well as the clothes and hand washing. Children who are not trained may play there with parents, but can only be left in the nursery. It was $6 per hour the last time I checked, and you can reserve 35 hours in advance....possibly add more after boarding if desired. Suggestion--DS MUST use the potty before you drop him off at the Oceaneer's Club. About 90 minutes later, you go down to see how he's doing and take him to the potty again. Repeat as needed. Thus, YOU are helping him to succeed. Kids are allowed to wear pull ups late at night, but not during the day.
IMHO, people bring water on board because they like to spend money or are fixated on bottled water. The ship's water is both distilled and then filtered. It tastes good. I bring one empty water bottle and refill it from the ship's taps for excursions.
To answer the question of a few pages back, I've been to Europe a couple times, but never to Spain. I finished my graduation requirements a couple months early, so did an 11 week backpack/hostel/Eurail pass trip many many years ago....then a trip to Amsterdam with DD a couple years ago. I'm sailing solo as DD is now in college. What else was I supposed to answer?
I'll be out of town for a week in February, THEN I can start thinking about April...well, not totally. I've done the hotel/plane/etc. stuff already. I just can't get too into it till I get the February school finished.