1)
Why does it say "Lo Yatza Chatzer"?
1.
Rashi (from Yerushalmi Sanhedrin 10:2): Hashem rushed to tell him, lest rumors spread in the city that death was decreed on Chizkiyah.
2)
Why is it written ha'Ir, and we pronounce it Chatzer?
1.
Radak citing Eruvin 26a: This teaches that the roads of kings are like medium cities.
