Why is there a 'Yud' missing from the word "Nesi'im"?
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
Moshav Zekenim, Ba'al ha'Turim: The Nesi'im who made Shalom between the Shevatim and Bnei Gad and Bnei Reuven were zealous for the Mitzvah, so there it is written Malei.
See Sifsei Chachamim (citing R. Bachye). However, they learned their lesson however when it came to the inauguration of the Mishkan , when they were quick to bring their beautiful gifts.
Targum Yonason (27,28): They brought the stones from the River Pishon. They brought also from Gan Eden, the fragrant spices, the olive oil and pure balsam oil (for the Besamim, the Menorah and the anointing oil, respectively).
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: They brought the former because the names of the tribes were written (engraved) on them.
Rosh, Da'as Zekenim, Hadar Zekeinim: They brought the Avnei Shoham and Shemen ha'Mishchah together, and each prince brought the stone that was designated for his tribe and wrote (engraved) the name of his tribe on it.