The one time we sailed on Disney out of Miami, last year. I do not recall them checking Port Arrival Times before allowing someone to enter the terminal. Doesn't mean they won't for the OP, but we are talking about a smaller ship, utilizing a terminal that was designed for, and regularly handles 4000 passengers or more for RCCL.
OP, I would suggest heading to the port around 11:00, as you planned. Just be aware that you could be waiting in the terminal for a little longer after they start boarding at approx. 11:30-11:45. When we went, it took about 30 minutes in line just to get through security and into the terminal. As others mentioned, your Port Arrival Time of 12:30 - 1:00 is tied to your Boarding Group Number. You will get a Boarding Group Card with a number when you check in to get your keys. I would expect that your boarding group will likely be somewhere between 15 - 25. Once they start boarding, groups are called usually every 2 to 4 minutes, sometimes less. Once most of the people waiting in the terminal have boarded, they switch to open boarding, they will announce this, and if they haven't called your group yet when this happens, you will be allowed to board when they announce open boarding.