Eric Definately Keep the plywood for a solid base, attach the your Edge strip with Glue & Biscuits to the top only If you are concerned about the differance of expansion and contraction Then Glue the mahogany down to the ply with silicone. Apply spots of slicone about every 12" in each direction let the weight of the Mah. top settle on the silicone. Make sure you finish the back side of the Bar top with the same material as the topside. You can apply a few screws to keep the top from creeping while the silicone sets up. Or you can use tape. Do not use clamps and if you use the screws don't set them down tight. This is the tryed and true method of preventing problems from the differance's of movement from differant materials.
Good Luck George