Why did he say "hami'Biladei Hashem"?
Rashi: Did I come without permission?! The Navi said "Yisa
This verse is more explicit! Why didn't Rashi bring it here? Here he holds that it is not a source, for shortly after (7:20) it clarifies that Melech Ashur will come, but he will be destroyed. Perhaps there, Rashi explained 7:20 like Malbim (that Melech Ashur in the days of Achaz will fall), or he held that Ravshake did not hear the latter verse (it is in a different Parshah). (PF)
Hashem did not say "v'Hashchisah"!
Rashi (Melachim II, 18:25): He 1 added this on his own.
Some explain that he did not base himself on an explicit prophecy. He extrapolated, or decided that he will fight Hashem's battle, or said so to scare them (refer to Melachim II, 18:25:1:2-4,6).
Ravshake spoke in the name of Sancheriv. It is not clear if Sancheriv added this, or Ravshake. (PF)