What does "Navol Tibol" mean?
Rashi (citing Targum Onkelos: It means 'You will wither'.
Rashbam: 'Your speech will become blurred, when you try to respond to so many people simultaneously, as will theirs, (as each one cries out and receives no response), since you do not have the strength to answer them one after the other.
Targum Yonasan: 'You will fall off (like a withered leaf falls off the tree).
Why did Yisro use the word "Gam" twice in his statement?
Rashi: To include Aharon, Chur 1 and the 70 elders.
Targum Yonasan: It is to include Aharon and his sons and the 70 elders.
Chur had already been killed when they worshipped the Golden Calf (See Rashi 32:6), which preceded this Parshah! See Sifsei Chachamim.
Why did Moshe judge by himself? Why did he not enlist the help of Aharon, his sons, Chur and the current elders?
Or ha'Chayim: The people only wanted Moshe to judge them, since they had more confidence in him (and in his Ru'ach ha'Kodesh) than in anyone else.
Rashi writes that Yisro said that also Aharon, Chur and the 70 elders will wither. Chur was already killed earlier, when they worshipped the Golden Calf (See Rashi 32:6)!
Riva, Hadar Zekenim #1, Moshav Zekenim #1: He meant that even if Chur were alive, and he and the elders helped, [all of] you could not bear the exertion. 1 This does not apply below (verse 23), "v"Gam Kol ha'Am