1)
Why does it say "Malet Amaltecha... Ki Vatachta Vi"?
1.
Radak (16): You did to Yirmeyah (took him from the pit) against the officers' desire. You are one, and the officers are many, and you trusted in Me, and acted against them. Therefore, I will spare you, even though you are from Tzidkiyah's household. They will flee with Tzidkiyah, and die by the sword.
2.
Malbim: I will save you via Hashgachah, for you trusted in Me.