We seem to know that black hole energy, and apparently event horizon geometry and topology, are functions of Mass, Charge and Angular Momentum. Area is a function of geometry and topology as well, which apparently controls entropy. Strominger, et al, can show that the apparent agreement of string states to event horizon in extremal cases. What is the study of diffeomorphisms from extremal black holes solutions to schwarzschild solutions called? What are routes to show string states describe schwarzschild black holes as charge and angular momentum are taking to limit zero?
The generalization of the Strominger-Vafa calculation of the extremal black hole entropy to the case of neutral black holes is not just about a "diffeomorphism" (which is a word representing just a trivial coordinate transformation in the context of GR).
It is a totally different problem - something that you were implicitly aware of at the beginning of your question, before you contradicted yourself. So the whole calculation has to be done from scratch while one cannot rely on supersymmetry because non-extremal black holes such as Schwarzschild cannot be supersymmetric.
Despite this complication, the correct entropy of various non-supersymmetric black holes has been calculated from string theory, too.