The ethics of plea bargaining

I wonder about the ethics of plea bargaining. Suppose the accused is guilty of a serious crime. In that situation, it may well be in his self-interest to cop to a lesser charge.

However, even that is unjust. If he committed a more seriously crime, he shouldn't get off with a lighter sentence. Perhaps, though, that can sometimes be justified by the fact that in a less than ideal world, we must often compromise. It's better to settle for something then settle for nothing if you can't get everything you want. 

Be that as it may, there are more troubling scenarios. Take a case where the defendant is either innocent or guilty of a minor crime, but the prosecutor threatens him with indictment for a serious crime. That generates several major ethical problems:

i) It creates enormous opportunities for prosecutorial misconduct.

ii) If a prosecutor doesn't have the evidence to charge the accused with an actual crime, then he shouldn't be charged in the first place, or pending charges should be dropped. 

iii) It engineers a terrible dilemma for an innocent defendant. A fateful risk assessment. On the one hand, if the defendant refuses the deal and the prosecutor either drops the charges or fails to secure a conviction, then that's better than serving time. On the other hand, there's the risk of conviction for an even more serious, trumped up charge, if the defendant refuses the deal. That's quite a gamble. 

iv) It coerces innocent defendants to confess to a crime they didn't commit. Under duress, they are made to lie by admitting to a crime, when in fact they committed no crime. That's straight out of Kafka. 

v) A basic rationale for plea bargains is to incentivize the accused to implicate someone higher up the food chain. While there can be some merit in that strategy, it's offset by loss of credibility if the accused incriminates someone else to save his own skin. Is an extorted confession of guilt credible?