In fact, what Polanski's defense boils down to finally is what you might call the Ezra Pound Exception, i.e., that some people consider great artists to be exempt from moral judgement (to wit, "So what if Ezra Pound was Fascist, he was a great poet, etc."). If you take Polanski's talent out of the equation, then his defense falls apart, which is easy to see with a little thought experiment. Assuming that the facts are not in dispute—that Polanski pled guilty to unlawful sex with a minor and then fled the country to escape sentencing—consider this: if he were just an ordinary 76-year-old man, and not the guy who made Chinatown and Rosemary's Baby and Knife in the Water, then no one would take the other reasons for not extraditing him at all seriously. Her mother was pushy? So what. She was "advanced" for her age? So what. It happened a long time ago? So what. For anybody else but a celebrated and/or wealthy guy, none of this would matter.
And even if you do think that Polanski's personal history and the victim's forgiveness are mitigating factors, surely the proper venue for taking them into account would be a sentencing hearing in Los Angeles County. If they are mitigating factors—and I honestly don't know if they are or aren't, that's not my point—then let Polanski return to face the charges he admitted to, and let a judge take them into account. Given that he confessed to the truth of the charges, and then fled the country to avoid facing the consequences, letting him off the hook during the extradition process just feels wrong to me, or at least premature.