1)
Why should Chizkiyah not fear?
1.
Malbim: They reviled Me, and not Yisrael. I will do (defeat Sancheriv) for My sake.
2)
Who are "Na'arei Melech Ashur"?
1.
Radak (Melachim II, 19:6): They are his servants, i.e. Ravshake and his colleagues. Malbim - even though now (the first time) they themselves blasphemed (they were not Sheluchim for this), Hashem will save Yisrael for their sake.
