1) how easy is it to get lost on the ship? - - - the Miracle isn't huge, so while you might take a minute to find your way to different venues, once you're on the correct deck, if you keep walking around you should be fine. When in doubt, as a friendly crew member for help. There's also a map of the ship in the app you can start looking at or print out and do a little studying of the deck plans to know where most stuff is. In general, aside from your cabin most things will be on the Lido deck up top, and on decks 3-4-5.
2) there seems like a lot to do I feel like I’m going to miss out? - - - you probably won't be able to do everything, don't stress about FOMO, plan the things you want to do the most and be flexible with the rest.
3) how long does check in take? - - - can depend, especially right now, but go at your check-in time (not earlier right now) and you hopefully will be on the ship is under an hour, sometimes much less.
4) I’ve prepaid everything (wifi, drink package, shore excursions etc) what can I expect to pay for on the ship? - - - stuff you might buy in the shops, casino, some drink taxes if you have drinks before leaving US waters, spa treatments, specialty dining/seafood shack...so all optional stuff.
5) we had to select a dinner time but no breakfast and lunch time? Is dinner different? - - - yes, your set dining time for dinner will be at the same time, same table, with the same servers each night. Breakfast and lunch have open times for the various places to eat and you can check-in to the dining room via the app and the buffet, Guy's burgers, and Blue Iguana are come whenever you want (when they're open).
6) is the dinner the buffet or ive heard the “main dining hall” is that like restaurant style?? - - - You can choose to eat in the buffet if you like, or you can go to your assigned restaurant at your dining time. The menus for the restaurant will be available in the app so you can decide where you'd like to eat.
7) is there a dress code for any part of the ship or time? - - - most of the time is just casual. No swim/beach wear in the dining room. And 1or 2 nights on your ship will be "elegant" night where most folks dress up a bit and they don't like you to wear shorts and t-shirts (though you can probably get away with it if you're not too dressed down). The app will always tell you the evening dress code.
8) where do you set your room key/sail and sign card when you are in the pool, going on the water slide etc I don’t want anyone to take it and use my sail and sign. - - - you usually can just leave it with your towels and stuff...they don't really have too many issues with people taking them and using them since they have cameras everywhere and would be able to track people down, plus the waiters can see your picture when you use your card. If you're really concerned, you can keep the card with you in a pocket or something. Water won't hurt them.