If working with lists, I've always found it better to specify the actual attribute you wish to use, such as "Title", so it would be _RequestedBy.Title and _Department.Title (or similar).
As far as working with the Parent Request, that can also be a little different as you are now working with a collection object (as there can be multiple parents (which does seem a bit strange in itself)) - I've always used a loop to access those items.