At the planar level, the Law-Chaos axis in D&D is a shallow take on randomness

[Mechalich]

I won't argue this too strenuously, but feel the need to point out that there are very serious Ph.D.s in physics who will disagree strenuously with you. Especially the last sentence: it is, in fact, impossible to "measure all current effects" to the degree required, due to the Uncertainty Principle (which has been demonstrated in experiment). So there is, in theory, true randomness buried in the quantum mechanics of things.

In our universe, sure. On Mechanus, again a literal clockwork universe, probably not. And one of the things about Planescape is that belief changes reality. So it's actually a goal of the Modrons, as a cosmic force, to eliminate all fundamental randomness - in the quantum mechanical sense or any other mathematical sense that might produce such randomness - from reality in order to render it perfectly ordered.