Combining And Accessing Multiple Youtube Accounts When 1 Is No Longer Accessible

Two Youtube accounts, both in the same name: The first created years ago by an employee using a now-defunct email address; which has a number of valuable videos. The second created later, which 'subscribes' to the first; so we can 'see' the first account's content.One video on the first account now has a copyright claim which is false, however we cannot start a claim process because we cannot either logon to the first account, nor "claim" it. When we try to claim it, it says that account/channel cannot be claimed.Equally logging on fails & wants to verify via the old email address which no longer exists.How do we rightfully claim and use the first account within the second, and then deal with the copyright issue?