Coicidentaly, I had to euthanise a small cory I purchased lately that had exactly the same erratic swimming symptoms discussed here whilst in quarantine. I also used the clove oil method and it was quick and hopefuly as humane as possible.
I'd like to make a distinction between using ice water and freezing. When you drop a fish in frozen water the shock will imeediately stun it and render it unconsious. Then, depending on the exact effect it had on the fish (which can be unpredictable), you may still have to actualy kill it or it may already be dead. It is, however, a quick, efficient and humane way to send a TROPICAL fish UNCONSCIOUS. It won't necessarily kill the fish and it normaly has little effect on coldwater species.
Freezing, on the other hand, is very inhumane. The process is slow and painful. If the fish is placed in a plastic bag - with or without water - befroehand, it may as well just be left to suffocate. If it is kept in water, the water will begin to freeze and the fish will gradualy and painfuly die. If the fish is still alive when crystals start forming in its body, you can imagine the agony it would have to suffer.
RIP Cactus the bronze cory. (and also RIP Seaweed - my little old zebra danio who died last night - she had been lethargic from old age recently...)