JRC,
You write, "... me perplexed in how you state things that would be self evident in pure mathematics. "
I am not sure what you are saying here. The only "self-evident" idea I have ever said in any discussion is "we can only know what we think we know". As corollary to this, we cannot know "what is" physical reality. But can only know our measurements, observations and understanding of our experience of physical reality.
It seems to me a core misunderstanding is what is a mathematical identity. And why Planck's Law as was and is is not a mathematical identity. But I am demonstrating and arguing it IS! So let me focus on this and forego your other points of agreement/disagreement for another time.
Simply, a mathematical identity does not depend on any physical suppositions in its derivation. Like, for example, the physical existence of "energy quanta" having energy given by hv used by Planck, Einstein etc. to derive Planck's Law. My derivation of Planck's Law does not make any physical assumptions. It is a pure math derivation. Thus, a mathematical truth.
As such, it describes the functional relationship between the "incremental change" of a quantity, its "average over that interval", and its "instantaneous value" at the start of that interval. Notice! Nothing here about physics, energy, frequency, etc. I show Planck's Law in its mathematical essence is just that!
You write, "... I don't understand why you said that Planck's Law could not be an identity because he introduced a physical constant". It's more than that. The derivation depended on the assumption of the physical existence of "energy quanta" having energy given by hv. Thus, it is not a mathematical derivation that does not depend on physical assumptions having a "physical view". Math does not claim any "physical view".
Certainly we can substitute in any mathematical identity quantities and equivalences consistent with the mathematical meaning of those parts in the mathematical identity. Thus, if we know or assume the "change of energy" is given as hv, and the corresponding "average energy" for this is given by kT we can substitute these values in my mathematical identity to get Planck's Law in the form familiar in Physics.
This is no different than making appropriate substitutions in the Pythagorean Theorem. But this does not make the Pythagorean Theorem any less a mathematical identity. Again, the issue here is the physical assumptions used in the derivation. A mathematical identity makes no such assumptions!
Constantinos