Again you are dodging the question and not answering:

Was there a shift in the users metadata?
What will you do with the funds in the end if the user does not send you the information you are asking for?

I am trying to find a solution, which the user could use without exposing his documents, and which would make sure he is the rightful owner.

You are able to send him his BTC back as there is no legal issue in doing so. You keep avoiding the point and are acting very suspiciously.