That makes sense! When the decision for azab has been made, praying for them is now haraam. But since Ibrahim AS made a promise, God made an exception for him.
The prayer of Abraham for the forgiveness of his father was only because of a promise he had promised him, but when it had become clear to him that he (his father) was an enemy to Allah he (Abraham) disowned him. Lo! Abraham was soft of heart, lone suffering. [9:113-114]
That would mean, however, that Ibrahim AS disowned him but carried on praying. The disowning was the tradition of the prophets. This disownment would usually mean that prayers are not made as in the case of Muhammad SAWS, but for Ibrahim AS an exception was made.
That is my working theory.
But there’s one problem:
Its this part:
but when it had become clear to him that he (his father) was an enemy to Allah he (Abraham) disowned him.
This is written in contrast to to:
The prayer of Abraham for the forgiveness of his father was only because of a promise he had promised him
So my theory breaks apart, because it means Ibrahim AS prayed before itmaam e hujjat, and then stopped after. So this is very confusing.
1) Either the prayer was made before itmaam e hujjat which should be no problem, prayers are allowed in this period. So why was it mentioned when the Prophet SAWS was being told not to pray?
2)Or the prayer was after itmaam e hujjat, which raises the question of why the above verses are written in the order they are.