Kskid, all of your questions relate to your local geology, and nobody not intimately familiar with it could reasonably be expected to provide answers that would apply to the pond you have had dug.

Nobody would be happy to dig into a sand vein but even that would relate to their extent.

If I were you, I'd wait to see if I had a problem before I attempted to solve it, particularly since you say that you hired a reputable local contractor.