For me, CSM always came out looking less smooth and even than 1708. If you go with CSM only, you'll be doing a lot more fairing.
With 3/4" ply, I think your option #3 is best. A layer of CSM is required with 1708; it already has mat stitched to it. An extra layer of CSM would add strength, but also weight. On top of 3/4" plywood, I don't think you need extra strength.