Yes, you can install the border over a landscape rock surface and fill it with playground material.
You would need 4 kits. On the two 15 foot sides, you would need to cut 1 ft. off the end of the timber to get 15 feet total length.
Yes that is correct. For a 16 ft. x 48 ft. area, that would be 8 kits.
2 kits would contain 8 timbers and joints and that will create a perfect circle, 10.5 feet in diameter. Since the timbers have a rigid curve to them, any more timbers used will not create a perfect circle.