The Kli Chendah on last week’s parsha raises the question (discussed already by the Pnei Yehoshua) of what the definition of melacha on Shabbos consisted of after the command to observe Shabbos was given at Marah. The definition of av melacha in our halachic world means a significant labor that was performed in the construction of the Mishkan (B.K. 2a, Tosfos). But how could that possibly have been the definition in Marah, pre-building of the Mishkan?
My off-the-cuff idea is that the observance of Shabbos as given in Marah was not a kiyum mitzvah of shemiras Shabbos per se (i.e. with all the associated gedarim of hilchos shabbos like avos/tolados), but rather was a kiyum mitzvah of listening to the words of a navi (i.e. Moshe Rabeinu). There was simply no concept of a cheftza shel mitzvah as we know it until mattan Torah.
Rashi writes that mitzvos were given at Marah “l’hisasek bahem.” The Brisker Rav is medayek that this implies an obligation of limud but not of practical observance, in contradiction to a number of gemaras (e.g. Sanhedrin 56) which imply that practical observance did begin in Marah. It’s a little bit of a stretch, but I am wondering if Rashi perhaps is alluding to this idea that what was given in Marah was something to be "involved in" (in the sense of practical observance as well as limud), but not what we could truly call a mitzvah.
I’ll save you the trouble of asking and raise the question myself: what of the commandments like taking the korban pesach in Mitzrayim – were those mitzvos or not? I’m not sure that’s a knockout punch to my theory. Maybe pre-mattan Torah these were not mitzvos in the same sense or the same form they took on post-mattan Torah. How can you prove otherwise?