So you're suggesting slowly freezing is ok because ice crystals can't form? I'm missing your point...
Freezing or cold-shocking are very, very inhumane methods of killing fish - certainly not 'euthanasia'. Both are very slow and, because fish are cold-blooded, they don't die immediately (though they may 'appear' to stop moving their gills) but ice crystals etc CAN form while they are still alive. Keep in mind that a fish' body is not the same as a warm-blooded human's - no crystals could possibly form in a human while he/she was still alive - but fish don't generate any heat.
Also, flushing fish is a bad idea because you risk introducing disease to local waters (or, if your fish were to survive, even introducing a non-indiginous species). Consider that aquarium fish have developed, relatively speaking, some resistence to the common aquarium diseases - wild fish won't have.
Only clove oil and destroying the brain are humane methods for killing fish.