I have read on this site the procedures for pouring new concrete over existing concrete slabs and understand the importance of making sure the new concrete adheres to the existing. Assuming the proper use of rebar, can approx. 2 inches of gravel be used on top of the existing slab to reduce the amount of new concrete (approx. 4-5 inches) that is necessary to achieve overall desired height of the new patio? Many thanks for any and all recommendations.

That is a lot of weight to add. Are you sure the footing in strong enough?