The connection between mercy and justice is as follows. Justice relates to the degrees to which a man can understand and act upon equality, fairness, liberty, freedom, reason,and knowledge.Reason and knowledge are prerequisites for equality, fairness, and freedom for without reason and knowledge a man's capacity for judgement is impaired.
Mercy follows upon judgement which is reasoned and knowledgeable. The more a man knows and understands about himself and the other the more he will be inclined to be merciful to the other.
Injustice is an evaluation. Since no man's judgement is perfect there will always be a measure of injustice in any evaluation. Injustice is relative absence of justice. It follows that a judgement with a bias towards mercy is a safer judgement.
Neither 'Reverse Injustice' nor 'Reversed Injustice' make sense in English. 'Reversed Injustice' sounds something like the overturning of a previous legal verdict however it's not a phrase I have ever heard or read in any context.