Can anyone explain the rabbit-starvation described by
Stefansson and other explorers? Is it just a result of
not eating enough Calories? Or "protein toxicity"?

A pound of 93/7 extra lean ground beef has approximately
90g protein, 30g fat, 630 Calories, 57% protein, 43% fat.
You'd eat 3 pounds (270g protein) to get 1890 Calories.

A pound of boneless skinless chicken breast has roughly
105g protein, 6g fat, 474 Calories, 89% protein, 11% fat.
You'd eat 4 pounds (420g protein) to get 1896 Calories.

Some popular diets suggest eating lean meat for short
periods to rapidly take off weight. Unfortunately, they
also risk eating up muscles, organs, bones, and other
vital tissues. Unless you eat liver, there is no source
of fat-soluble vitamins from a lean meat diet.

So, does rabbit starvation result from
1) Not enough fat / too much protein
2) Not enough calories in general
3) Depleting vitamin / mineral reserves

I don't agree with others here who have said that you
won't suffer rabbit starvation if you eat the whole
rabbit - bone marrow, organ meats, visceral fat, etc.
You would still not have enough calories, unless you
added some fat and/or carbohydrate.