1)
What is the Chidush "you will speak My words to them"?
1.
Malbim: Do not omit a word from My words due to fear.
2)
Why does it say "Ki Meri Hemah"?
1.
Radak: They are Beis Meri. Similarly, "va'Ani Sefilah" (Tehilim 109:4) means Ish Tefilah.
2.
Malbim: Whether they hear you or refrain, do not think that it is better not to say a word that will not be heard, and it is better that they be Shogeg than Mezid. Even without this, they are rebellious and Mezid!