We forgive for hurt that we've suffered, so, of course the father can't forgive for the sufferings of his son. He forgives for his own suffering which was the loss of his son.
As far as the argument that "none is due," you're right. But being right is sometimes a very unpeaceful and lonely place to find ourselves.
When Christ on the cross asked God the Father to "forgive them for they know not what they do" He certainly knew that his murderers hadn't asked for forgiveness so, by your terms of justice, didn't deserve it.
By your definition of deserved forgiveness, Christ was wrong to ask for this... unless there is another reason for practicing forgiveness.
Christ was correct and that there is, indeed, another reason to forgive. That reason is love - the kind of love that God has for all men, which is unconditional and, therefore, selfless.
Within this second reason is another, not so obvious, reason to forgive whether the person "deserves" it or not. That reason is our own balance of emotional and mental health.
By forgiving others, we are freed of the burden of carrying those injustices which, eventually, wear us down even to the point of crippling us emotionally.
I think that the Bergs are Jews. Torah suggests that "forgiving" those who harm us is presumptuous. Forgiveness is not ours to give since we are not privy to the secret hearts of our oppressors.
Jesus, a Jew, did not call out his personal forgiveness of his betrayers and murderers, he called out to G-d, our Father, to do what only He can do, forgive.
Our task is to not become embittered nor to seek revenge. But we are well advised not to commit the sin of pride by assuming the role of "Forgiver" so that we appear "divine" when in fact, we are simply unwilling or unable to pray for the strength to endure in faith, as Job did.
I find this public ritual of "forgiving" someone who has taken the life of your child to be shocking, and from my own viewpoint, singulary suspect.