This is stupid. There is no such thing as “proving the documents are declassified”. Under the Presidential Records Act whatever documents the Presidents deems as “personal records” are released to him regardless of classification when he leaves office.
A US District Court already ruled on this in the case of the Clinton audio tapes of White House conversations that he kept in his sock drawer. They were ruled personal records and Judicial Watch was denied access to them via a FOIA request.
I’m just playing devil’s advocate here. Did Trump ever claim these as personal records? He should have done it right up front when they first started asking them returned. I’ve said all along the President has had terrible legal advice.