WASHINGTON (Reuters) - A White House plan to stick the Cabinet-level Office of Trade Representative in the Commerce Department could raise some tricky questions for Jeffrey Zients, a senior aide to President Barack Obama who was a key architect of the plan and is now seen as a front-runner to head that very agency. A former U.S. trade official called Zients' potential nomination to the top trade post "ironic," while a trade policy source said it would be "like putting the wolf in charge of the hen house. ...