I still say never. There are also cases of Cannibalism of perfectly well-fed people, who are just insane. Extreme circumstances? Sure not. Cannibalism is insanity. It is not even healthy, human flesh is slightly toxic for humans. There is no natural instinct that you can call upon.
And stop repeating the same three examples (and you also forget Stalingrad, which was a far harder case of starvation and suffering, with surprisingly little documented cases of cannibalism). It doesn't work - for each case you have, anybody here can drag out millions of cases where humans did not do it. The people who commit cannibalism are not the norm - they are strongly differing to the norm, thus fulfilling many qualifications for being insane in the modern world.
Just like people take the death of their dog as hard as the death of their own child, without being insane. It might not be within your limits of sympathy, but that doesn't say that they are insane. What counts is only the emotional connection, which can be even very hard for cars, you might be surprised how hard sane people can take the loss of their car. You are maybe hardly the reference for sanity.
Also, that you would eat the dog of your parents, if this is your "only" chance, already suggests that you won't look very hard for "other" chances. You are right, people who still hope don't commit cannibalism. But people who committed cannibalism did actually do it out of the hope for surviving long enough for being rescued. Again, your hypothesis does not hold water. People who loose hope are less likely to violate their own ethics for survival, they will try to make the best of their situation. Like the passenger on the Titanic, who, rather than fighting for his rescue like a wild animal, sat down in the saloon with a cigar and a bottle of champagne, calmly awaiting his fate in the best possible way. (Yes, extreme situations can back fire for deducting anything about the normal life)
Please realize: Your models of the reality has first of all deal with the normal cases, before you can stress-test them with the extremes. If you start your epic gyro compass on living among homicidal cannibals, you might be right with the assumption that it would be wise to be on the top of the food chain. But that is not normal. That is the extreme exception, that won't work outside the closed envelope of the model.