Hello. After seeing many essays filled with anti-Platonistic prejudices, I tried to follow your reasoning to see if it can be taken as a good reference for a defense of Platonism, but I found it both hard to follow and in some aspects disappointing. Of course the main idea is clear and worthy, that anti-Platonism is itself a metaphysical presupposition and it has many troubles. The problem is in the details. The very fact of being hard to follow is a weak point for an argument to convince people who are not fond of mathematics and are naturally tempted to skip technicities that look not so clear (indeed for the concern of anti-Platonist essays of the present contest, authors usually base their denial of mathematical realities on their natural dislike towards mathematics, and thus towards any formalized kind of exposition), and people are usually only ready to "accept conclusions by faith" in doing so when it does not conflict their basic philosophical convictions.
I am usually not afraid of technicities when they are needed, since I am mathematician. However, technical details also deserve their own kind of clarifications, that is, they need to be appropriate. And more precisely what puzzles me with your very formal way of writing, is its strange combination with what seems to be your exclusively philosophical background, that is, you seem to have only learned the topic as "philosophy of mathematics", a branch of philosophy in the way taught by academic philosophers (and you even mention your degree to be only in the philosophy of physics, not mathematics), and not as "mathematical logic" taught by professional mathematicians.
And the problem I found with academic philosophy and its diverse branches of "philosophy of science" such as the "philosophy of mathematics" and "philosophy of physics", is that they usually keep a poor understanding of the fields they are pretending to discuss : they poorly understand science, which does not even mean they better understand its philosophical aspects, since philosophers usually keep a fuzzy and unreliable ways of reasoning in their own field. Because they usually focus their study on traditional ideas and what other "science philosophers" wrote, texts are supposed to be about the philosophical aspects of science, and ideas that previous philosophers took note of as philosophical. In doing so, the ideas of science philosophers often remain quite outdated with respect to scientific progress, ignoring what scientists actually discovered just because it is not written "Philosophy" on the title of their works.
I guess this explains why in your attempt to defend Platonism in mathematics, you did not even say a word about the Completeness theorem of first-order logic, which originally was Godel's PhD thesis, and which I see as a terrible (but usual) omission in the debate. You only mentioned his later Incompleteness theorem. Why ? Maybe because you only read other academic philosophers teaching about the incompleteness theorem because it turned out to be more "famous" among non-mathematicians, while they failed to notice the no less fundamental importance of the Completeness theorem, actually a cornerstone of mathematical logic (no less famous than the incompleteness theorem among specialists), for their "philosophical debates", where they seem glad to picture things in a "philosophical" style, that is a coexistence of competing "viewpoints" supposed to be antagonistic, irreducible and equally defensible, with no chance of any rational resolution. But whose viewpoints are these, seriously ? Mainly those of philosophers and a few fringe mathematicians, it seems, while the overwhelming majority of mathematicians, even specialists of mathematical logic, does not feel concerned by these "debates" in their works anyway.
And specialists of mathematical logic have a good reason to no more show themselves in arguments between opposite "philosophies of mathematics" as expressed in terms of the old debate between Platonism, formalism and so on, because this old debate turned out to be outdated in the light of the more recent understanding of mathematical foundations. This way, the new scientific updates fall out of the radar screen of professional philosophers, who keep repeating the old debate without noticing that they are waiting in the wrong train that is not going to leave because the right train already left.
Now to try analyzing the contents of your arguments on Platonism in mathematics :
Your whole discussion seems contained in the century-old formulation of foundational problems as expressed by Hilbert's program, that is, the search for a formalization of mathematics, when the global picture of mathematical foundations was not well understood yet.
You seem to express the question of Platonism in terms of the correspondence between "a given mathematical edifice E" that is not yet formalized, and a formalized version of it, Sy ; and you seem to define Platonism as the idea that E preexists before Sy, while being essentially different from it. Sorry but I find this quite a strange way to define Platonism, I would rather be tempted to see this claim as better associated with anti-Platonism than with Platonism.
What the heck is E ? Non-formalized mathematics, you say. But precisely I don't see the sense of trying to make any logical reasoning about such a thing as "non-formalized mathematics". Because as soon as some "non-formalized mathematics" is regarded as an object of discourse, it becomes reified, analyzed... and finally turns out to be a sort of formal system itself ! Otherwise if a "non-formalized mathematics" (whatever it might be) fundamentally differs from any formalized version, then in which sense can it be considered any kind of perfect mathematics independent of human beings at all ? I do not see any fundamental structural difference between "non-formalized" mathematics and formalized mathematics, and I do not see the issue of Platonism as having anything to do with the hypothesis of such a difference. Where is the contradiction between mathematics being pure and perfect, and being defined by a perfectly rigorous formalization ? I cannot see any.
As for me, I did not waste time "learning philosophy" but followed the more recent theories of mathematics and physics, from which I cared to both further clarify the expression of theories from a purely mathematical viewpoint, and directly identify their philosophical dimensions without the "help" of philosophers. And the picture I found this way of philosophical aspects, is quite different from what philosophers usually say. Before my work, I would have called myself a Platonist. Now the evidence I found from the (mathematical) foundations of mathematics led me to a sort of intermediate position (as was mentioned in comments about Cantor, which I did not check), close to Platonism but with an important difference : I found that the mathematical universe in itself is not fixed but has its own time, independent but similar to our time !
You can find my ideas and findings on these issues in my web site, especially the introduction and the philosophical aspects page.
Now this understanding of time in the foundations of mathematics also brings light by analogy, to the role of time and its irreversibility in physics. As I see you also interested both about irreversibility in physics and the interpretations of quantum physics, you may be interested with my essay A mind/mathematics dualistic foundation of physical reality where I propose a solution to these issues.