1)
Why is it written Yadata, and we pronounce it Yadati?
1.
Radak: The Kesiv refers to Kel - You know the Rasha's intents for me. Do Din (judgment) for me from him! The pronunciation means that I know that Hashem does Din for the poor. So He will do for me, and take vengeance for me against the Rasha!
2)
Who are "Oni" and "Evyon"?
1.
Rashi: They are His servants; He will take vengeance for them.
2.
Malbim: "Oni" is David, and "Evyon" are his men.