Well work and schedules delayed this for a couple weeks. Finally got the tops installed this morning. The plywood was 3/4 and fit flush with the bottom of the trim edge so no wood is visible. The marble has some veining so yes the backs were reinforced. It took me and a helper to gently lift them in place and set. Drilled the holes and I'm off to HD soon for a couple install supplies including a stone sealer for the marble (if that's even needed) and the stone sinks.
Now comes the backspash. We're going to open a bottle of wine tonight and start "brain storming". I am planning to use a combination of tile from the counters all the way up to the ceiling. I will then frame the mirrors with moulding to finish. If anyone has creative ideas feel free to submit a pic and/or description.