The IPCA finally got back to me, and they do investigate Police processes and can and do accept complaints about them. Their website has been updated, they have a new brochure and the new SOI looks clearer.
So this is no longer necessary, I'm not quite sure how to handle ending the life of a topic tho… :)
The whole process has, however, convinced me we need to extend the OIA to the IPCA (or remove their exemption) as it shouldn't've taken five months and this much chasing to get an answer, and the only mechanism to address their repeated non-responses was their Minister. I shall go add it to the Open Government topic.