Hey Dragonfly, I know exactly the problem you are experiencing. It's a moving water table created by the granite you are sitting on. The front of my house in the summer has no water table at 5' until the fall and then it is at about 2' until the next summer. There is absolutely nothing you can do about this except move your pond above the water table that is there now, or build up the ground level to compensate for this.
As to the thickness of winter ice....no more than 8" where I am and also in Ottawa.
Good luck on your project, and don't let anything stop you from building what is the most soothing thing in nature......a waterfall and fish in a pond.