Can we just go to the rooftop bar for a cocktail or two and watch the sunset? Do they ask for admission or not?

Yes you can, on the 16th Floor bar / restaurant.

You can go up. When I went on 09/01/2018, the 16th floor and first floors are the only floors you don't need a keycard for. You can have a drink by the pool or even swim if you want, you don't need to be a guest. They also have complimentary wifi, no password/login needed (at least that was the case on 09/01/2018).