(a) To say that the Rambam assumes that R. Akiva doesn't hold of Halcha L:Moshe Misinai means that he contradicts his own view that there can be no Maclokes about Halach L'Moshe. Although Chavas Yair quotes many such examples, I am not aware of an answer for the Rambam.
(b) Why can't R.Akiva hold of Kroi and still agree that it's modified by Halacha L'Moshe? The Gemora's kasha"Kroy lama li" is vice versa.
meir levin, Brooklyn,NY
(a) You have raised a good point. Generally our answer to reconcile the Rambam is that we concede that in the details there is room for arguments even in Halacha L'moshe M'sinai; as for the main point, though, there is no argument (see for example the Perush ha'Mishnah at the end of Eduyos).
So too here, even though R. Akiva argues and does not hold of the Halachah l'Moshe mi'Sinai, he does not argue on the main point: that we must add on to the seventh year (before it starts). He learns it, though, from a pasuk and not from Halachah l'Moshe mi'Sinai. It seems likely that the Rambam would agree that the may be an argument among the Tana'im whether the Halachah is learned from an explicit verse or from a Halachah l'Moshe mi'Sinai. The intention of the Rambam is to affirm that there is no contention in the basic Halachah l'Moshe mi'Sinai (as is clear from the Rambam's words, in his Introduction to the Mishnah).
(b) According to your suggestion, why did the Gemara not accept the words of Rav Nachman and suggest that Rebbi Akiva could hold from both Halachah l'Moshe mi'Sinai and the verse? It seems clear from this Gemara that if the Halachah is learned from a Halachah l'Moshe mi'Sinai the Pasuk becomes superfluous, and must be used for another Drasha. (We did write something similar to what you suggested in the Insights, though, in the name of the Nachalas David.)
-Mordecai