We’ve done CC many ways, including beach chairs, excursions and cabana.
Considering you’ll be at the cabana no earlier than 12:15 if everything goes smoothly, without having grabbed a lunch, I would say no. You’ll get maybe 3 hours to use it. It wouldn’t be worth 643$ to me. I would just grab beach chairs and do some activities around the island.
To answer your first question, it comes with a safe, fresh fruits, chips, granola bars, a hammock, a private shower, a changing area, access to a private part of the beach, snorkeling equipment rental, bike rental, sunscreen, tube and water mat, taxi service to the tram (a 5 min walk at the most) and butler service.
It is almost impossible to get a cabana without being Concierge, especially now that they’ve converted cabins on Magic and Wonder.
DCL truly favors Concierge guests for those. On our last cruise, we were rerouted to CC for a surprise double dip. The asked all the Concierge guests if they wanted to keep their previous cabana on the original day, replace it with one on the new CC day or get one both days. Then they asked Concierge guests who had not reserved one if they wanted one of those available on either day before considering the waitlist setup at guests services. They also keep one available when bookings open in case a Concierge guest forgets to request one.