That would be multiple Executive Office of the President staff members who provide eyewitness testimonies at trial - assuming it gets that far - verifying that he had declared everything being shipped to Florida was declassified.
As for signing documents, the President is only going to sign the politically important documents. Signing of a bunch of routine declassification notices doesn’t’ fall into that category. Some staff member who has been delegated that power would sign/should have signed them. If it didn’t happen before 12:00 pm on 21 January 2021, it’s a real shame but hardly treason or espionage - on anybody’s part.
What trial? There are no pending charges, and no trial on the docket. And if Trump and/or his lawyers are refusing to provide until a possible future trial any evidence, of any kind, that Trump ever declassified these documents, they're going to lose on that issue in this court. There's just no way around that.