Why is there a 'Yud' missing from the word "ve'ha'Nesi'im Hevi'u ... "?
Rashi: When it came to donating for the Mishkan, the princes decided to wait until everybody had given their donations, and to fill in whatever was still needed. However, when it came to the crunch, what the people gave was more than enough, and there remained for them to bring only the precious stones for the Choshen and the Eifod. And it is because of the Nesi'im's laziness 1 that the Yud is missing from their name. 2
Targum Yonason: Because it refers to the Clouds, 3 which brought the stones from the River Pishon, 4 and the olive oil and pure balsam oil (for the Besamim, the Menorah and the anointing oil, respectively).
Moshav Zekenim and Ba'al ha'Turim: The Nesi'im who made Shalom between the Shevatim and B'nei Gad and B'nei Reuven were zealous for the Mitzvah, so there it is written Malei.
See Sifsei Chachamim (citing R. Bachye) and Or ha'Chayim. They learned their lesson however when it came to the inauguration of the Mishkan , when they were quick to bring their beautiful gifts. According to Avos de'R. Nasan, chapter 11, this Pasuk (notwithstanding the missing 'Vav), is actually speaking to the credit of the Ne'si'im - 'When Moshe was discussing the Mishkan, he refused to consult the Nesi'im (See Torah Temimah, note 21); so they took the initiative and brought the Shoham and onyx stones and the commodities mentioned in the next Pasuk.
Which are also called Nesi'im, as in Tehilim 135:7 & Mishlei 25:14 - PF.
Which they deposited with the Nesi'ims' portion of Manna in the morning (Yoma, 75a).
Why did the princes donate specifically the Avnei Shoham (the onyx stones) for the Eifod and the Avnei Milu'im (the filling stones) for the Choshen?
Rashi: Refer to 35:27:1:1*.
Rashbam, Rosh, Da'as Zekenim and Hadar Zekeinim: Each Nasi brought the stone that represented his tribe on the Choshen 1 (and the two Avnei Shoham and the Shemen ha'Mishchah they brought between them - Rosh, Da'as Zekenim and Hadar Zekeinim).
Avos de'R. Nasan, Perek 11: When the Nesi'im saw that Moshe did not consult them about building the Mishkan, 2 they became upset at not being able to participate, so they brought the extra donation of the Avnei and Avnei Milui'im ... .
