a single error of judgement is hardly enough to convict for involuntary manslaughter.
Oh yes it is. Happens in courts around the world day in and day out and I am referring to reputable legal systems here.
As far as I can see the CVR and FDR data is material evidence at a potential crime scene - I see no reason why it should NOT be made available to police investigators.
Here you run into the conflict between accident investigation and criminal investigation. The legislation surrounding aircraft accident investigation requires the handing over of such data. However, in a criminal investigation this would amount to overruling the defendant's fundamental right to silence. This is why in a lot of jurisdictions there is the protection that such data cannot be released for purposes other than the accident investigation.
Should there be a ruling that the data is to be released to the police I would imagine that there is going to be a very quick appeal lodged and it will be challenged up the judicial chain. In the meantime you can obtain an order that the data is not to be released pending an appeal.