It emerges that when Moshe said "ושמעו מצרים" this was not a reason not to kill them but as an opening from where Moshe had the right to daven. The tfilla itself was actually said in the pasuk "ועתה יגדל נא כח השם" and the invocation of the 13 Middos Harachamim which follow seamlessly from the words ושמעו מצרים. Now that he has the right to daven he does so by invoking the 13 Middos of Rachamim.
We can also answer the question of the Ohr Hachaim why it says ושמעו with an extra "ו" when it should have said "ישמעו מצרים" [see there]. Based on what we said maybe the explanation is Moshe was saying that earlier on You wanted to prove your rulership over the world to the Egyptains but now they are going to doubt it, so what's the point. "ושמעו מצרים" After all Your miracles ... and now they are going to hear. The "ו" connects to what happened earlier.
We can also understand why Moshe first mentioned the Egyptians and then the Goyim in general. He started with the Egyptians because their story is what gave him the right to daven in the first place. He then continues that the fact is that ALL the Goyim [including the Egyptians] are going to doubt Your power and say that You are powerless against those who dwell in Eretz Yisrael [as Rashi says in פסוק ט"ז].
