1)
Why did he pray? Hashem already said that the baby will die?
1.
Radak: Hashem could decide to be gracious and spare the baby, like David said (verse 22). Even though it was a punishment for his sin, he received [other] great punishments - the sword will not depart from your house, v'Shachav Im Nashecha. He prayed to remove the punishment of the baby.
2.
Malbim: Tefilah can help even after a decree. Even though the decree was a punishment for his sin, he thought to cleanse his sin through affliction and fasting.
2)
What is the meaning of "u'Va v'Lan v'Shachav Artzah"?
