dxrs0- Welcome to Cruise Reviews.
1) Yes any shore excursions you purchase onboard will be added to your onboard account. The group leader of the shore excursion probably will give a demonstration for beginning snorkelers. You do not have to stay at Stirrup Cay all day. Tender boats will be running back and forth all day and you can come and go as you please.
2) You will have beach towels in your cabin or pick them up by the pool. If those towels aren't returned they will be charged to your account. These might not be the same towels with the NCL logo. They usually have them for sale in the gift shop.
3) If you made your own hotel reservation and didn't book it through the NCL program then they will not pick you up at the hotel. NCL has a program where they use certain hotels for pre/post stays and if you book one of their hotels in the program then you can get their transfers. It usually is very easy to take a taxi from any hotel.
4) TSA requires you to be onboard no later than 90 minutes before the ship leaves. I usually check in for cruises anywhere between 11AM-12:00noon. Rooms usually aren't ready until 1PM. You can check your larger luggage at the pier with porters and carry your smaller carry-ons with you. You can check-in and go right up to the Lido deck for lunch at the buffet.
You will have a daily bulletin in your cabin every day that will tell you where and when all the activities and events are happening. Don't stress, everything is quite easy. People and staff are very friendly on cruises. If you don't understand something there's always someone who will help you.