Well, an update to this post is
long overdue!
In late March, while cutting and burning brush on the part of the pond which had been mostly backfilled by then, my brother and I saw a rat jump into a hole in the ground. Lo and behold, we found a 6" clay overflow pipe! We dug at intervals and traced it back to a penetration in the buried culvert near the road. This discovery greatly simplified the plan forward for drainage (or seemed to at the time).
Overflow Inlet - 6" Clay Pipe
Overflow Outlet