I'm trying to follow your logic, or lack thereof. First you say that the SIU gleans its investigative information third hand from the police and it is vetted before being turned over to the SIU. This has been demonstrated to you as blatantly untrue. Your response is a description that subject officers retain the right to not give a statement against interest, which of course does not address the assertion that all investigative information is vetted by police lawyers and given to the SIU. If they refuse to give a statement, there is no information to be forwarded, and/or vetted. By the way, if a subject officer does wish to give a statement, it is not submitted by the subject officer after being vetted by a lawyer. The subject officer gives an in person viva voce statement to the SIU.
On a side note....I generally refrain from being a grammar nazi, but I find it ironic if not somewhat telling that you refer to me as demonstrating "ignorence". First of all, name calling is the last bastion of those unable to construct and maintain an intellectual argument. And for the record, it is spelled "ignorance". I won't denigrate you for your error, I will just revel in the humour contained therein.