I agree that freezing will slow themetabolic rate, but it won't necessarily kill them. If you put a large crayfish or lobster in the freezer for an hour then take it out it will start moving again after 10-15 minutes.
I think the reason for not stabbing it in the head is actually that if you then boil it, the water will get inside the carapace and it will go watery inside. For crabs it is *very* easy to sever the brain with a point of strong sharp scissors. Lobsters and crayfish are slightly more difficult. We would ice lobsters for 90 minutes to 2 hours before making any cuts.
Whilst boiling appears to be the most cruel, my boss tells me that if the water is already boiling and you only do one animal at a time the drop in temperature from adding the crab/lobster/crayfish is sufficiently low to result in death in a few seconds. Again, he would pre-ice it first.