Dear Phillip,
I just read your short essay and agree that mathematics in some sense "exists" in some realm for which "mathematical mindscape" is as good a label as any. In my view, part of the reason that some people have problems with platonism is that we use the same word, existence, to describe the being of mathematical objects, which is however, of a very different kind than the being of physical objects. Ideally, one should come up with a different word to denote these different senses of existence.
It is very perceptive of you to identify that what sets being in the physical sense apart from other others is almost always its close association with a particular location. Mathematical objects, dreams, ideas, perceptions and feelings seem to have in common that they lack such an association.
(As a short digression, note that the mainstream view that objects characterized by v=c, which are almost universally taken to physically exist, do not have rest frames has the probably unwelcome consequence that it lumps such objects together with those mentioned above, which is one reason I find this view inadequate [Instead I would say they have no spacetime rest frames, which is a different kind of statement even though under the current paradigm it sounds the same]).
I think your identification of the factor that both physics and mathematics happen to be formulable in terms of rules that can be applied across the fields is a big part of the picture that explains how they relate to each other, but I feel that you did not go far enough with your argument. Specifically, when you say "In summary, if there is no true randomness, then every process is determined by rules." I would reply that even if there is true randomness (and QM suggests that there is) every process is determined by rules, for random processes the rules may not be deterministic but we can still formulate probabilistic rules.
I do not think that the argument you gave exhausts all possible explanations for the effectiveness of mathematics in physics but does supply an important ingredient. Your writing style is also very clear and accessible, and this is greatly appreciated.
Best wishes,
Armin