If you believe, as I do, that such a person is guilty of a crime, then we're not risking the false guiltiness of an innocent person. At best, we're risking their level of sentencing. And I'm open to the idea of reduced sentences for AI CSAM, but it shouldn't be a factor in determination of guilt (i.e. it should be a matter between the judge and the defendant, rather than something the prosecution needs to prove).
With regards to CSAM criminalization in general, there is a real risk of punishing innocent people that may have been framed by planted evidence. But this is a risk regardless of whether the evidence is "real" CSAM or not, so legalizing possession of AI-generated CSAM doesn't reduce the risk of an innocent person being framed. It might make it "easier" for a bad actor to frame someone, since they can now do it with AI content instead of real content. But if they're already planting evidence, do they really care whether they're committing a crime while preparing the evidence? And besides, if we keep the AI content illegal, then it's equally legally risky to frame someone with it as it is to frame them with real content.
The problem of prosecuting "innocent" people, whether you believe they're innocent because they were framed or because they're only guilty of possessing AI-generated CSAM, should be addressed at the time of enforcement. Stop using entrapment and fishing expeditions as an enforcement mechanism. Only open investigations when they start with a real and identifiable victim, rather than a potential perpetrator.