1) Top Sail Lounge is YC-exclusive
2) Ask nicely at the YC concierge desk, it'll be up to the YC director. I've seen videos / photos from folks who've said they've been allowed in for a quick tour, but it's by no means guaranteed.
3) Depends on what country you've booked in - some include, some don't. If you've already booked, check your booking on the website, on the Booking Details tab it'll show "Service Charges - Gratuities included" if they are. If you haven't, it'll tell you during the booking process. Note that that's for the daily gratuities only; anything you spend on-board gets a 15% gratuity added.
4) Yes, there's an alcohol-free package. I see the UK pricing, as that's where I am - for a 7-day cruise, £147 per adult; kids get it for less, but mine are beyond that now so it doesn't show me the current price. Includes
Non-alcoholic cocktails
Fresh fruit and protein cocktails
Energy drinks
Sodas
Energy drinks
Flavored water
Soda by glass and bottle
Wide range of hot drinks
Soft serve ice cream
Beverage gratuities included