There are many examples on houzz of contrasting counters for perimeter and island. For example butcher block on island and stone on perimeter or a solid color on one and something with movement on another. I would only mix two stones if one had movement and one was fairly solid. The stainless on perimeter and something else on island sounds good to me. Here is a pic of stainless counters with what looks like a stone breakfast bar. (I hope I understood your question right). http://houzz.com/photos/3137660

How about butcher block instead? IKEA and the big box stores sell it for countertops, the price is decent, and you won't have as many issues with buckling, water damage, or what to do with the edge as you would with the ply. If the edges of the ply get wet, or if any seam around a sink gets wet, the plywood will "blow" and you will have to replace it all, anyway.

Water marks don't need to be cleaned off (unless they are permanent). It just means your stone needs to be sealed. You can get a high grade sealer/ impregnator at Granquartz or Regent. Seal your marble twice a year and that problem should go away. Buy one of the half liters or 4 oz bottles (the former should last you two seals at least). This is a good quality for marble: https://www.granquartz.com/Pages/ItemProfile_Detail.aspx?ItemProfileId=1748&ItemClassNo=92 And this one is particularly good for sealing marble, especially in the kitchen: https://www.regentstoneproducts.com/item_detail2.aspx?ItemCode=ZETASEAL

I think what you have done is amazing - so fresh and so modern - it is a great example to people like myself who have an old kitchen with good bones but no longer modern. So many people spend far too much money ripping out usable kitchens, that function well but they think they need to get the new and latest, often just to keep up with the "Jones". You have done an amazing job without the huge cost of replacing all your cabinetry. WELL DONE.