I direct your attention to the physics SE question on Philosophical Interpretation of String Theory. Verbatim questions from the post:
How to explain all the mathematical structure that arises in string theory?
How does one know what one derives is math and what is physics? Where is the boundary?
The first is extremely broad; string theory incorporates many aspects of mathematics. Secondly, the 'boundary' between whether something derived is mathematics or physics is primarily opinion-based.
The user has added other questions, and I feel other than being too broad and opinion-based in some aspects, the post should be split into separate questions. For example, one discusses the possibility of string theory as being some effective field theory, or approximation of a more fundamental framework, which is not related to the two questions above.
Unfortunately, a bounty has been offered, and the question cannot be closed (at least not by normal users). How has this question slipped passed us for two days, and not been closed?