For better or worse, these figures aren't quite as significant as they seem, because a lot of alumns restrict what their donations can be used for. E.g. the Widener one for Harvard's main library, which was fantastically restrictive even for that, and also required a swimming test since a son died in the Titanic disaster (not that being able to swim would have made much difference in water that cold).

So, yeah, the interest could pay for all the tuition, except X of it is restricted to the law school, Y to the med school, Z to arts and sciences but not in any way that it can contribute to tuition, etc.

Ah, and tuition money is particularly valued, because it's unrestricted....

It could, except that Harvard already uses a lot of the interest on that endowment to cover expenses. If they didn't have the endowment, tuition would have to be a lot higher.