Tha Grand Dad. after years of believing that TUs were bad I have changed my mind.
Your union rep is not in your command chain although they could potentially be in the future. With the present in-house system things can be a little too cozy between the brass. Only time you might win is if the person you are complaining against is a potential target for the brass.
One problem with the service investigating itself and finding itself to blame is the matter of compensation. The only method that I can see at the moment is to get a solicitor pdq.
Maybe there is scope for a 'forces friend' legal group retained through a small subsscription by all officers. Then whenever you feel you are being hung out to dry . . .