hmmmm good question, regarding the transfers, you need to ask your TA if Celebrity sails out of Miami, if they do then you need to ask your TA to find out if you can even get a transfer from their Miami terminal to their Ft. Lauderdale terminal. This way you would be guaranteed that the ship would wait if the bus got delayed for any reason. This could be a pain in the neck but at least you wouldn't have to worry about missing the ship.
Look into cab and limo costs, just allow yourself enough "wiggle" room to get to Ft. Lauderdale. Get the earliest disembarkation you can from RCCL>