What this prosecutor said has no bearing on the merits of the case against the Indian diplomat. He did not "confess" to subverting the Indian justice system. That is merely your spin.
The US negotiated with the Government of Pakistan (GoP) and gained the release of Raymond Davis. If the GoP was confident in their "justice" system, and in the guilt of Raymond Davis, they would not have released him. The fact that they released him shows that the charges were political and subject to political negotiation. As an Indian surely you understand the reality of Pakistani justice. Their system is political, not judicial in the classic sense.
Given the massive public sentiment at the time in that country, it is clear that they released him because of intense pressure from the US. not a lack of belief in their legal system.
The US also claimed that he had diplomatic immunity. Yes, immunity.
Anyway, I cannot be effed continuing this.