The primary difficulty is that the spirit of the law, as the dictates of conscience, are regarded, if not simply as subjective, that is, as arbitrary, in practical terms they are viewed as ideological or politically motivated. So, the further one goes from the ideal form of a positive law which requires no moral judgments in its administration, an ideal which is never realized and unrealizable, the more one becomes politically violent and arbitrary (as we see today in the American legal circumstance and elsewhere).
As I mentioned in my, quite unfortunately, and wrongfully, suppressed answer to the last question pertaining to mercy, these questions are sterile without a specific content.
Justice is usually distinguished from law, or, more exactly, from positive law. Justice has to do with conscious of the gods, or moral conscience. With the subject matter of the Euthyphro for example. The Flintstones cartoon has a show where Fred's moral deliberations manifest as an angel and a devil on his shoulders. Ergo, the gods speaking within, inner deliberation of what Cicero calls the ego arbiter. This can be distinguished from utilitarian external considerations based on experience which can be made wholly clear and legally objective as standards.
Is "blind Justice" supposed to mean the "letter of the law?" Then it names positive law. Rather than Natural Law. Natural Law appeals to Justice as measured within. By the wise, or, at least, practically wise (phronimos) judge, or the judge of sound normal faculties of heart and mind.
As to "Mercy for the merciful" and the other two statements, this is a peculiarly curious principle. In a certain sense, so far as I can see, it is original (not withstanding its biblical overtones). However, what is required is that the notion of mercy be given specific content so it can be worked out if the ideas are practically absurd or sound. That is, do they have a sense in real life, and what is it?