Certainly the type of critter that pulls down a big duck could be a species other than a fish and in that case all the pumping and chemicals won't help (reptile attack?)

I agree better sampling before nuking the pond is wise. Baited jug line should catch a catfish over time. Large LMB would be on the list of fish that would ambush a duck in such a foolhearted fashion.

If the critter is still in the pond it/he/she may take another pass at a big lure that acts the same way on the surface. You said this same critter kept taking passes as the other ducks. A very large wooden lure mimicking a mouse or frog on the surface is pretty hard for a big LMB to leave alone. In northern lakes a pike or musky would be happy to swipe at a duck but I don't think that is on the list of likely species in your pond.

The other thing that is odd about your description is that she seemed to have been pulled down by 'one leg' I would think a large bass would try to use vacuum to suck in the body of the duck and I don't think a LMB can grip one leg with its mouth and hold it down.

Toothy predators (pike, pickerel, gar pike, musky) are more likely to use their jaws to clamp on a leg and pull it down. I would say this technique is also favored by alligators.

If you did decide to start pumping down the pond and were successful you may be able to spot the large fish as the water levels go down and all the remaining creatures are concentrated in a smaller area.

A one time treatment with alum or water clearing pellets might really help. If the visibility was suddenly 3 feet down or more you more easily could spot a large fish.