Very picturesque setting Bill...I see why you're so bummed!
Couple more questions that might help;
Did your builder work the soil in the ENTIRE pool area originally. Did he work the NEW shorline when the levee was raised?
You said you found sand/gravel in the levee that came from the pool area. This means you had drainage seams in your soil. These seams could be anywhere and if the new, higher shoreline was not worked/compacted, more seams could be draining your lake into the watertable. These seams could be running in any direction.
You might want to do a telephone consult with Mike Oto, co-owner and dirt Guru extraordinair of Pond Boss ottosdirtservice.com
Considering the expenses you have already laid out along with the frustration of time, effort, and the extreme difficulty of finding a leak....I would suggest letting the pond leak to it's lowest point, then draining it a few more feet and allowing it to dry a few weeks. After the exposed shoreline is dry enough, get the heavy equipment in there to work the exposed shoreline and use a sheepsfoot roller to compact the entire shoreline and dam face to seal the entire lake.
It won't be cheap, but wherever the leak is, it will be at or above the low draining point and this solution will fix it.
PS
You can click on my screen name to send me a private message or my website to get my phone number.
