" a very old dream, and one that is probably unattainable both because of Godel's theorem"
Gödel's incompleteness theorems are often invoked as an argument against the possibility of complete and consistent axiom set from which all interactions at all scales of physical reality can be derived. The problem is, the incompleteness theorem apply to the formulation meta-mathematical statements about systems (arithmetic principally). But one has to remember that while, aside from basic rules of composition, there are constraints to the making of such meta-mathematical statements, theorems, (nothing prevents false statements or statements that can't be derived from any given finite axiom set), physical reality strictly constrains any system so that it must be consistent with the fundamental laws that govern forces and other interactions. So does Gödel's incompleteness theorems really preclude any possible answer to Hilbert's 6th problem?
If the Universe is made of a finite set of fundamental objects which combine in accordance to a finite set of laws that a finite number of fundamental interactions to produce physical reality, then it follows that Gödel's first incompleteness theorem is, at least in its present form, wrong when applied to reality. Also, if you believe that the fundamental components and laws are a consistent and that the Universe is a coherent system, then a physical interpretation of Gödel's second incompleteness theorem must also be wrong.
"in what way do these axioms and theorems exist, and where do they exist? Are they Platonic forms for example?"
We need to distinguish the axiom from the fundamental aspects of reality they would stand for. And by definitions, axioms cannot be proven. They are merely defined. Once that is done, the axiomatic system may be put to the test. If the axiomatic system is complete and consistent, then all that interactions should be derivable from it. It should also enable the emergences of falsifiable predictions.
" what decides the form they have? (there are various possible forms of logic: who chose this one?)"
That is the tricky part. Any choice must be made based on a number of assumptions. There can be a number of viable axiomatic systems that may be used, but whatever the choice, it must be self-consistent an all interactions must be either derivable from or reducible to it.
"how do they have the power to create any physical entity whatever? Actually axiomatic systems are rather limited in their powers and in their ability to represent reality."
If the Universe is found to be both consistent and complete, that is, the fundamental particles and the laws that govern them are consistent (consistency) and all that they produce remains part of the Universe (completeness), then all physical processes are emergent. It can then be shown that it is possible to create an axiomatic system that represent the fundamental aspects of reality and that representation of all interactions can be derived within such axiomatic system. If the Universe is a consistent and complete, then axiomatic representation can certainly be powerful enough to represent it.
Though a work in progress, I believe that I have shown that an hypothetical universe that is comparable to our Universe in complexity can emerge from an simple axiomatic set. My essay, titled "Questioning the Assumption that Space is Continuous" shows one way that can be done (my essay is based on a larger work, part of which can be freely).