Based on my not-a-lawyer experience, it is very likely that at every stage the prosecution team was going for exactly what they though they could prove in court at the time. They can't predict what evidence may come out in the future, and they can't predict how Judges will interpret the facts.
There were many victims. Not prosecuting one doesn't mean they can't for the others.
They reneged on the deal, and a conviction with a prison sentence was the result. I thought "scumbag" was the more polite option, frankly, but what word would you prefer? Not that I'll not stick with "scumbag", but I'm always open to new, perhaps more creative options.
Which innocent person are you talking about here?
I feel this captures the extent of what you're saying, unless we're making an implicit claim about "using the word innocent in this context has X,Y,Z societal side effects", etc etc
<insert standard disclaimer>, but yeah, let the guy walk; and disbar the scumbag prosecutor while we're at it. Disbar if not for being a scumbag, but for being grossly incompetent if nothing else. IANAL, so maybe that's why I can't see how anyone involved thought this would end well.