The wounds don't look exactly like those that I would expect from a heron, but the location of the front wound, near the spine is fairly common with heron attacks. Wounds by heron often cause paralysis which sound like what your fish is experiencing. If it is heron, then it would explain the other fish going missing and this fish is not likely to get better. Look for heron very early in the morning, first light is common. If heron, then it knows where there is a buffet, and will continue to show up for more until all that can be eaten are gone. Put nets above the pond making it impossible for them to strike through, or high enough to make it difficult for them to find a quick escape path.
Zone 7 A/B
Keep your words sweet. You never know when you may have to eat them.
Richard