I work for a small software development company, and I just wanted to weigh in that in some situations, granting each user complete administrative access to their system is an absolute necessity. In a previous job, we were on a Windows domain and I needed to contact university IT to install anything. Especially web browsers -- the most important tool in my job as a web developer. It was a huge hassle, resulted in lots of lost time, and that actually was a small part of why I moved on (didn't feel trusted).
Obviously, this does not apply to all industries, but kudos to the OP for at least trying to make it easy on this new person.
I completely agree with keeping a backup. It's more important than being able to access the drive on that laptop. In my current position, all work is saved in a remote Git repository (aka "in the cloud") or on our NAS. It's part of our workflow, we don't email files, we get them from the NAS, so there's no way for an employee to keep work files on their machine locally without a remote copy. There is literally nothing on any of the employee machines that is not copied anywhere else.
For two people, you could look into using Bittorrent Sync, Dropbox, or buying a network hard drive. This way, if the person changed their FileVault encryption keys (it's fairly easy to do) or deleted your account, you would have a copy of their work somewhere else.