Doros Rishonim (4:13): The Rambam’s words which are focused on the general principles of the nature of Tradition and the interpretation of the verses (drashos) indicate that the drashos and the format of the drashos as well as the given and take of the gemora regarding the drashos don’t constitute proof that the halacha under discussion is a Torah halacha. The proof whether a halacha is a Torah halacha can only be determined within the context of the discussion in the gemora. If analysis indicates that it is a Torah law then we conclude that the tradition is that this a Torah law. However if the analysis shows us that this is a rabbinic law then we can not conclude the contrary from the fact that the gemora is describing it as being learnt from a drasha of Torah verse. That is because the Tradition is foundation of everything. Therefore in this case the drasha and limud is only serving as an asmachta (memory device). Thus the status of a halacha as being from the Torah is not determined by how it is seems to be derived - but rather that is what our Tradition says. Thus if our Tradition says a halacha is rabbinic – it is rabbinic even though there are many interpretations from Torah verse. The form of apparent deriviation does not add or subtract from the status established by Tradition that it is rabbinic.
Doros Rishonim (Chapter 11): Concerning all the dershos, they are only to provide support for a halacha which is known by Tradition and the derasha is simply to show that there is nothing which is not alluded to by the Torah. They are also used to support a halacha which has been decided from concepts found in the Mishna or from a Tradition transmitted by their teachers or because of logical reasoning based on the concepts of the Torah [but the derasha is not the source of the halacha].