I agree with your idea of lining the hillside with compacted clay.
I've already suggested this, but given the expense and frustrations you have had, I would line the entire pond with a foot of compacted clay and be done with the worries. I don't recall you ever saying how large your pond is. If you have good clay in the sandy hillside, you could borrow there and rework that hill along the shoreline and make damn sure you seal that sand seam also.
You may have seams in the bottom of the pond basin, especially in that old creek bed that is leaking as well---it's quite common.
Get'r sealed then you can spend your future pond $'s on the fish I'll be happy to bring to ya!
Which ever way you go, I sure hope it seals for you!
