What a lovely setting you have! I'm not sure why you are sinking pots into the ground, but I suspect it's because you are trying to grow plants that don't grow well in sandy soil. Personally, if this were my land, I would plant native grasses, native forbs and native plants directly in the sandy soil. I think you are creating a garden that will require alot of maintenance. Every year you will need to pull the pots and either root prune or transplant into larger pots to allow the root systems room to grow.
Unfortunately I'm not familiar with the plant material that grows where you live, but I did a google search with the term: native plants Lake St. Clair Ontario and there were many sites to look at.
I'm not sure what you mean by using the shells in your gardening endeavors, but you can certainly use them as a decorative mulch or add some to the compost pile.
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.