kabbalas haTorah and mesorah

The Pnei Yehoshua points out a seeming contradiction between the following two gemaras:

1) The gemara (Megillah 2) writes that nevi'im instituted using final letters mem-nun-tzadi-pey-kaf that are different than the regular letters.  Asks the gemara: we have a principle that a navi is not allowed to create a new law; how could they invent this new din?  Answers the gemara: the concept of final letters existed in the past but was forgotten.  The nevi'im were not instituting a new law, they were just re-establishing past practice.  (See Sukkah 46 for a similar idea.)

2) The gemara (Temurah 16) writes that 3000 halachos were forgotten in the days of mourning following Moshe's death.  Asks the gemara: why didn't Yehoshua or Shmuel use nevuah to rediscover those lost laws?  Answers the gemara: a navi has no right to be mechadesh new halachos.  The laws were only restored through the brilliant pilpul of Osniel ben Knaz.

The sugya in Megillah allows for a navi to use the power of prophecy to re-establish laws that were previously known and forgotten, yet the sugya in Temurah tells us that nevuah could not be use to rediscover the halachos forgotten upon the death of Moshe.  How do you resolve the contradiction?

Rav Wahrman in his She'eiris Yosef vol 4 suggests that the two sugyos are addressing two different problems, as the principle of ain navi rashay l'chadesh davar is two dinim in one: 1) the navi cannot use prophecy to rule on halacha; 2) prophecy is not a cheftza shel Torah, but is in a category of knowledge all its own. The gemara in Megillah is addressing the second problem only.  Since final letters existed in the past as a cheftza shel Torah, the navi is not creating new law.  The gemara in Temurah is addressing the former issue, and dealing with the right of the navi as a gavra to pasken using nevuah.  Whether or not these halachos existed in the past is irrelevant to this question, as it is an issue of process, not outcome.

Rav Wahrman (link) in a few parenthetical lines snuck into the middle of the piece offers a possible second answer, which is the one I want to focus on.  He writes  that in order for something to be a cheftza shel Torah it is not enough for it to have simply been received by Moshe Rabeinu.  Torah = mesorah.  For something to become a cheftza shel Torah it has to be passed on to future generations.  The nevi'im who lived generations after kabbalas haTorah were restoring a tradition that had existed once upon a time, but been lost over the years.  However, the halachos that were forgotten immediately upon the death of Moshe were too new, too fresh, to have yet been incorporated into the mesorah.  They were lost right out of the starting gate, at generation 1 after kabbalas haTorah.  When there is no mesorah to start with, a navi has to right to create a new one.

In light of this chiddush I would suggest that the Mishna in Avos that tells us "Moshe kibeil Torah m'Sinau u'mesarah l'Yehoshua" is not telling us about two events, namely 1) kabbalas haTorah and 2) its transmission to Yehoshua, but rather is telling us about one event only: kabbalas haTorah.  What the Mishna is telling us is that a kabbalas haTorah is a true kabbalah only when there is a "mesarah..." that follows -- when the kabbalah impacts and effects the future.

Rashi in Pesachim (68) writes that the chiyuv simcha of Shavuos is  "...l'har'os she'noach u'mekubal yom zeh l'Yisrael she'nitna Torah bo."  R' Shlomo Zalman Auerbach (posted here) was medayek from Rashi that the simcha of Torah that we celebrate on Shavuos is something that we have to show to others, to demonstrate.  Perhaps if kabbalas haTorah is complete only if Torah is given over to others, the simcha of kabbalas haTorah is complete only if we give it over to others as well.