Does he tell all of Yisrael to repent?
Rashi: He tells Yehudah to repent.
Malbim: He addresses all of Yisrael other than Shomron.
Why is this written here?
Rashi (citing Sifri Balak va'Yeshev Yisrael ba'Shitim): He tells Yehudah to repent, lest it be punished like Shomron. A Mashal for this is a city that rebelled against the king. He sent a general to destroy it. The general was expert and settled. He said, take for yourselves days 1 - if not, I will do to you like I did to Medinah Plonis and its companions, and to county Plonis and its companions.
The Vilna Gaon's text of Sifri is 'words to answer (appease) the king.' The Netziv explains, take days to find an answer...
Why does it say "Ad Hashem Elokecha"?
Rashi citing Pesikta d'Rav Kahana Shuvah 164: Repent when He is Od (still) Hashem - Midas ha'Rachamim. If not, He is Elokecha - Midas ha'Din. Your Advocate should not become your Prosecutor!
Radak: "Ad" is like El. The same applies to "v'Shavta Ad Hashem Elokecha" (Devarim 4:30), "Shuvah Adai" (Yo'el 2:12).
Radak citing Yuma 86a: Great is Teshuvah - it reaches until Kisei ha'Kavod!
Malbim: "Ad" implies that Hashem is afar, and you must go to Him until you reach where He is. I.e. you must abandon your prior sins and regret the past, that you erred, and seek the path to return to Hashem.
What do we learn from "Ki Chashalta ba'Avonecha"?
Rashi: Stumbling blocks came to you due to your sins.
Radak: You see that you stumbled due to your sins - therefore you should return to Hashem! Nothing will raise you from your stumbling other than returning to Him.
Malbim: You did not sin in the way of rebellion. You stumbled via Malchei Shomron. You are like Shogeg or Ones, which can be fixed.
