I take it we talk straight here and in good faith, so I hope that nothing I say is taken personally in a negative sense.
It goes like this: I am not going to make half enquiries from Avis as anonymous, that does not work and is not right. Instead I would put my full weight in any dealings with officialdom (name, contact, reasoning) in an up front manner. For reasons that I will not give here I am not willing to do that , and so it must be to others to make that effort if they choose. It is not asking websleuths in particular to do this.
Secondly, we are going round in circles here. The family want all done to find the pair, the police are advising method I presume, and are not answering online calls for registration number, presumably because they have info that it is not now relevant. However the case is presented as having no leads, and the family are at a loss to what may have happened , in which case registration number is a main information to provide to the public. We do not know that registration number is not relevant.
Why would anyone assume from all of that that providing registration number is not relevant, instead of it being a kind of official formality between forces that it not be issued for whatever reason , for example ?
The possibility that there is some kind of face saving going on while people suffer for it is outrageous. What is more, any possible criminality of the two is completely insignificant to me beyond the information it brings on finding their whereabouts.
Finally this all reflects on how the case is being handled, it points to who knew what and when. This is important for family members in future when they try to analyse what has happened.
As I mentioned originally, I will just keep noting down anything that might be useful here, hopefully within the bounds of reason and in a form that will not cause distress to family members.