2. Those three foods combined have a full set of protein... something something something... Those are the magic three plants that go together for sustenance. Something about the proteins in them, but I'm too lazy to look it up right now.
3. I think it's been pretty well shown that people can thrive on a very wide variety of foods. Corn is the only obviously non-primal item there, as beans are in the gray area and squash is heartily approved.
The "running it off" is a balance between carbohydrate intake and insulin levels. Running that many miles will deplete your muscle glycogen and require you to intake far more carbs than an average American. I'm not sure what causes death among the marathoners that you reference.
I find it interesting that the Masai and Tarahumara have similar life expectancies, plus or minus a bit.
Finally, being exceedingly good at ultramarathons does not necessarily mean that these people are as healthy as they could be.