1)
Why did he say "v'Atah"?
1.
Metzudas David: Nebuchadnetzar did not overpower due to his strength, rather, due to Hashem's anger at their sins. You warned them to make peace with us. You are blameless. Therefore, I release you from your fetters, and you may do as you want.
2)
What is the meaning of "Pitachticha"?
1.
Rashi citing Targum Yonasan: I untied. This is like "va'Yfatach ha'Gemalim" (Bereishis 24:32).
2.
Rashi citing Menachem: It is an expression of opening, and so is "va'Yfatach ha'Gemalim" (Bereishis 24:32).
3)
What is the meaning of "v'Asim Es Eini Alecha"?
1.
Ramban (Bereishis 44:21): I will do good to you.
2.
Metzudas David: I will supervise over you.