Hi,
We just came back from our cruise this past Sunday. So maybe I can clear a few things out for you!
1- There is one Captain night and another formal night... so two in total.
2- The excursions talk will occur on the first night in San Juan and you will get the info through your TV. As for debarkation talk, since it's not like other ports, it will be done on the last day around 5hpm. Was very informative!
3- The boat drill occurs at 9h15-9h30, just before we get ready to leave... so no you cannot leave afterward. But you have until 8h30 to board back if you decide to go ashore.
4- In San Juan there is a possibility of self assist but the debarkation is very smooth and very well coordinated. We were out by 10ham and if people had flight early they were being let out first. Our flight was at 2h55pm. So no worries, we took time to eat and go back to our stateroon to get our luggage. We had plenty of time. Also, taxi ride is about 23$ for a single trip to the airport from port including 4 luggages. So cheaper than Carnival transfers!
Hope it's help.
BTW if you like snorkeling, we suggest you go for the Champagne Reef Snorkel in Dominica... beautiful and truly interesting.