I think OP has already gone through one cycle of to & fro with INZ. Probably they can rely on below statement and get statutory declaration but its always best to check with Case Officer if that's what they expected on the first place.
If police certificates are unavailable
If you cannot get a police certificate, provide detailed information of your attempts to get one. If we are satisfied that police certificates are not available or it would be unduly difficult for you to get them (for example where the authorities of any such country will not generally provide such certificates), we may instead ask you to make and provide a separate statutory declaration in both English and your own language.
If a statutory declaration is required, it must detail your attempts to obtain a police certificate and state whether or not you, or any accompanying family members, have been found guilty or convicted of, or charged with offences against the law in that country. The statutory declaration should also be corroborated by other information attesting to your character.