The operations manual could be considered to be the law but only for operations within that airline, they could in fact write any requirement in that they like but of course it has to be approved by the CAA before the AOC is issued.
Thats what I said DFC, you cannot get an AOC without a published and approved operations manual.
You are very lucky that no one has challenged the situation. A problem could arise if your check pilot stops someone before they even get off the ground and that person asks for the authority that they have to comment on another qualified pilots ability, what training they completed and to what standard they are certified.
DFC you are obviously very knowledgable but where have you got that from or how do you come to that conclusion?