"An alternative approach would be to allow such evidence at trial, and severely punish investigators who breach rights. ... However, a reluctance to punish high-status investigators means this approach could just result in more breaches of rights."

What if you allowed the convicted person to sue the investigators? That mitigates the reluctance to punish investigators -- or at least, the reluctance to begin proceedings against the investigators.

The convict could sue to get X years deducted from his sentence, and the investigator would have to serve a percentage of X depending on the egregiousness of the breach.