What is the significance of opening his mouth?
Radak: You will tell Bnei ha'Golah, this is like I prophesized to you, and you did not heed and believe my words.
Malbim: Just like Yechezkel told the people in the morning that his wife will die 12 hours later [at night; this hints to Churban ha'Mikdash], so Hashem will tell him the night before the Palit comes.
Why does it say "Es ha'Palit"?
Rashi: It means Im (with) ha'Palit.
Radak citing Targum Yonasan: With the coming of the Palit.
Why does it say "va'Tedaber v'Lo Se'alem Od"?
Rashi: Before I told you "v'Ne'elamta v'Lo Sihyeh Lahem [l'Ish Mochi'ach]" (3:26). From now, they will see that your words are Emes. You will no longer be silent from prophesizing to them.
Radak: When all your Nevu'os come to them, you will be able to speak to them and rebuke them. They will not be able to contest you at all.
