Why did Moshe not know the Halachah?
Rashi #1: He forgot the Halachah because, after appointing judges in the time of Yisro, he said to them "Whatever is too difficult for you, bring to me ... " 1
Rashi #2: This Parshah was fit to have been said through Moshe, and it is because the Bnos Tzelofchad were meritorious that it was said through them. 2
Targum Yonasan: Moshe actually knew the answer, and he claimed not to know it to teach the heads of Sanhedrin who would follow him not to be ashamed to say 'I don't know'. 3
Rashi writes that he forgot the Halachah because he told the judges "what is too difficult for you, bring to me." Why did Rashi not say so about killing the Mekoshesh and Megadef?
Perhaps this goes together with Rashi's second Perush; Bnos Tzelofchad merited that it be said through them. (PF) 1
Moshav Zekenim left this difficult; he understood that each answer suffices by itself. Or, also there, we could say that they 'merited 'that it be said through them. He could have asked why Rashi did not say so about Pesach Sheni. (PF)
Why is the Nun in "Mishpatan" big?
R. Bechayei: It hints to a Chacham great in Torah. One should not decide a Torah law in front of someone greater than himself. Rather, he brings the law in front of him.
Minchas Shai: There are Nun (50) gates of Binah (understanding). Moshe merited all but one of them (Nedarim 38a). 1
Shlah (Ta'anis, Torah Ohr 101): Yehoshua ben Nun brought Yisrael into Eretz Yisrael, whose air makes people wise, therefore he merited 50 gates of Binah. (I heard from R. Uziel Milevsky that the gate that Moshe lacked, pertained to Halachos of Eretz Yisrael, e.g. inheritance. Chizkuni - Moshe knew that daughters inherit when there are no sons. He did not know whether or not they inherit Ra'uy (property destined to come to their father) like Muchzak (what he owned when he died). Perhaps Moshe's doubt was only whether the inheritance of Eretz Yisrael, which was unique (refer to 26:55:3:1), was Ra'uy! - PF)