I'll throw my hat in the ring on this one.
You are the man on the ground, so only you can make a sound judgement, so i won't agree/disagree with anything you have written.
This is what GN3 says about it.
C1, Danger present now. These must be brought to the attention of the responsible person/user/client etc immediately, and isolation and/or rectification is the be RECOMMENDED without delay.
C2, Is not dangerous at the present time, but if left unattended, it will become a C1 sooner rather than later, and rectification is advised ASAP.
C3, This is only to be used where improvement is advised or recommended.
C1 or C2, cannot and should not be used just because the installation does not meet the current version of BS7671.
The example i use is RCD's. If the install has no RCD's, and it wasn't a requirement at the time the install was first commissioned, then there is no obligation to fit them now. If the installation met the standards required when it was first installed, it was safe then, and if the only thing wrong with it now, is that it doesn't comply with current regs, then there is nothing wrong with it, and it remains safe for continued use. You can only ADVISE improvement for things like installation of RCD's and use the code C3.
It is well explained in both BS7671, and GN3. You can't force change or opinions onto people. I am not trying to imply that you or anyone else is trying to do this, but when using these codings, they need to be fully understood, as we are giving an expert engineering view of the installation, so we need to adopt giving expert advice to keep the trade professional.
Cheers………….Howard