Do any commentators or other sources address the rationale of דבריו becoming דברו in Daniel 9:12? I’ve so far been unable to find any explanation for this.
Daat Sofrim on the verse writes:
"ויקם את דבריו. ממשיך לדבר שלא במישרין לה' מפני שמדובר על רעה. הכתיב הוא "דבריו", הרעה היתה קיום לכל דבריו אשר דיבר ועדות שכשם שנתקיימה הרעה כן תתקיים הטובה. הקרי הוא דברו, הדבר שדיבר בעניין הפורענות. בלשון זו התפלל שלמה לטובה: "ויקם ה' את דברו אשר דבר" (מלכים ח', כ')
Translation: "He carried out his threat. He continues to speak indirectly to Hashem because this is referring to bad things. The written form is "דבריו", the bad is a fulfillment of all that He had said, and a testament to the fact that as the bad came to be, so will the good. The reading form is "דברו", that which He spoke about regarding the retribution. In this manner Shlomo prayed for the good: "And the LORD has fulfilled the promise that He made" (Kings 1:8:20).
In other words, the plural form refers to everything that God said would happen, both the good and the bad, with the bad becoming a sign that the good will also happen, while the singular refers to that specific retribution that came upon them (that Daniel is speaking about in his prayer).