Staining: there is a clay product you can purchase at commercial stone shops. we have successfully used the clay to take out several stains. Unfortunately I don't remember the name.
Sealing: on some stones we have to put on 8 coats of sealer before we can get water to bubble on the surface. We use a product from
www.miraseal.com. It is expensive and worth it.
3/4 vs 5/8 plywood: with 3/4 you may still see a thick c-hair of the wood. I know you want to use 3/4 but I'd still double check with your fabricator to see about 5/8.
good luck,
dale