Moshe's added day (II) - divrei sofrim and dinei derabbanan

As we mentioned yesterday, the gemara (Shabbos 87) seems to want to have its cake and eat it - Moshe is said to have added a day of preparation before matan Torah 'mda'ato', based on his own insight, yet at the same time the gemara quotes a derivation for the addition from a hekesh, which indicates that it was implied by Hashem's command. Which way is it?
Tosfos tersely writes that the hekesh must not be a real derashsa, but Tos does not explain how else to understand it. Though Tosfos does not use the term, it is tempting to think of the limud as a type of asmachta. The MaHaRaL (Tiferes Yisrael 27) points out that we find in Chazal that dinim derived from derashos are categorized differently than dinim spelled out by a pasuk - the former are referred to as 'divrei sofrim' in the Mishna (Sanhedrin 87; the Rambam often utililzes these categories, a famous example being his categorization of kiddushei kesef as divrei sofrim). Although the additional day Moshe added is revealed by a derasha, it falls under the ruberic of 'mda'ato' because all derashos are categorically different than actual statements of the Torah. If so, why is this halacha in particular called 'mada'ato' more than any other divrei sofrim halacha? The MaHaRaL answers that other divei sofrim are all just a 'peirush' of a pasuk, but not part of the actual text and command of the Torah. However, here Hashem acknowledged that Moshe's understanding was not just a 'peirush' of the pasuk, but was part of the inherent meaning of the command. There seems to be a fundemental difference between the the way Tos and MaHaRaL understand use of the 13 middot: do the middot reveal the derived law as inherent in the text itself (Tos), or are the 13 middot interpretations of the text but not part of the text's inherent meaning (MaHaRaL).
According to Tosfos, if indeed the Torah did not command this extra day or the 2 others ideas of Moshe, what gave Moshe the right to act based on his understanding? Where was the legislative license for these enactments? R' Elchanan Wasserman in Koveitz Divrei Sofrim derives an astounding chiddush from here. He answers that there is no license needed for any Rabbinic enactment, and there is indeed no specific command in the Torah to follow any din derabbanan. The reason these enactments carry weight is because Moshe (in this case) and the Rabbis (later in history) are able to intuit the 'ratzon Hashem', and knowledge of the the ratzon Hashem even sans explicit command creates a sense of obligation. This is a mouthful to digest and needs some analysis - to be continued......