There is no simple answer to this question. There will often be crime scenes from which either complaints arise or where an officer discharged a firearm etc. In such a situation the police Investigating Officer and PONI Investigator would have an interest in the scene. The general agreement is that the more serious incident takes precedence. This has worked well but does require the Senior Investigators on the ground to agree forensic strategies and the approach taken.
No, we have no power to do so. But there is an onus on the PSNI to preserve a possible crime scene and any potential evidence. Any evidence that may relate to officers allegedly involved (e.g. uniforms, batons, notebooks etc.) that may be required for the investigation must be secured.