As admin, I assigned a Member entry to myself, but after completing that action I was unable to re-assign that entry back to the member. In fact the member's screenname doesn't even show up in the list of Authors anymore. How do I give that entry back to the member?
You will need to goto Members > Member Groups > [The member group the member is a question of] Edit Group > Scroll down to Member Account Privileges. Look for "Include Members in PUBLISH page multi-author list?" and select "Yes".
You shouldn't need to assign the entry to yourself as an admin though you should be able to edit it fine either way.