1)
Why did Hashem say "Tzofeh Nesaticha"?
1.
Rashi: Why are you silent? I made you like a lookout, who is stationed on a tower to see if troops come to the city. [If they come,] he blows a Shofar to warn the people to gather. So you should warn 1 [Bnei ha'Golah] to guard from punishment, lest I bring it on them!
1
Malbim: However, you do not warn about human swords. Rather, if you hear a decree of punishments, warn them from Me to repent!
2)
Why does it say "Mimeni"?
1.
Radak: [Warn them] in My name, with the words that you heard from Me.