Small edit - the coarse sand was spread on top of the existing black dirt, not sand.

to be honest - I don't follow your description or understand your goals.

Decomposed granite will wash into the pond, or under the liner if that is where water will end up going.

If you want a flag stone edge, the best way to do this is to put a row of man made square stones under the flag at the pond edge. The liner should go under these stones, and up behind the stones. These stones will give the flag edge a solid footing. the flag can then be cemented onto the stones for a nice solid edge.

Flag stone edging with crushed stone filling the spaces is not a great solution right next to the pond, since the spaces will be washed out. Mortor would be much better.

Hans,

This is late as I see you had your labor coming back around the 23rd.
I did something that has an outcome similar to what madtripper recommended. I put down a row of 'paving brick' around the outside edge of my pond where my 'coping stone' was going to go (yellow mark in pic) and ran my liner up over the top of them. The coping stone (about 4" thick) was set inside the pavers slightly overhanging the pond and I used mortar in between to keep them in place (pics). The water level comes about half way up the coping stone.
Don't trim any excess liner off so if what you're doing doesn't work out you can make some changes later.

Good luck,
Witz
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