Proof is the static part of logic (as, for instance, a proof that the circle cannot be "squared"). There is no temporal element in a formal proof. We approach such proofs as though all of its pieces are simultaneous. It is only when embodied in actuality that a proof moves according to the anterior-posterior aspects of its logic.
Validation is then the act by which the universe (qua logic) proves its consistency.
Validation means "To attest to the observation of" - (my definition)In the case of the universe at large, some element (which is not functionally attached to a given interaction) must "observe" an interaction and attest to its validity by performing some action such as assuming a new state.
The difference between validation and existence is entirely semantic.
So if I say, "Why must existence validate itself?", it is the same as saying "Why must existence exist?".
And that answer is explored further along.
Validation is then the identity of existence not a mere aspect of the case.