1)
Why did He need to say "Hispalalta Elai"? Obviously Chizkiyah prayed to Hashem!
1.
Malbim: Your Tefilah was primarily for the honor of My name.
2)
What is the meaning of "El Sancheriv"?
1.
Radak (Melachim II, 19:20): This is like due to Sancheriv.
