1. I've seen a couple, although I'm not sure where they had come from. Those strollers had very wide wheels - no other way to traverse the sand.
2. South beach has a sharp depth drop-off about 2 feet into the water. I would explicitly not go there with a child.
3. Yes, the ocean may be cooler, and no, pools are not heated.
4. Can't help here, however in general RCCL counts every head separately.