God, that was dire, and not even that entertainingly so- is Julian Fellowes on drugs???
He should have just jumped the shark shamelessly and got Hugh Bonneville, Bates, Rose and co to dress up in stripy burglar costumes and balaclavas with bags marked SWAG. Mind you, the rationale for why the family needed to get involved in the Incident of the Letter anyway was entertainingly tenuous. Dimwit Rose was a bystander. How useful it is to have a murderer/forger/ pickpocket valet, when one is a Peer of the Realm 
I used to complain that all the human elements of the the kitchen maid/footman love triangle/quartet/whatever were indistinguishable, but Mary's suitors are heading the same way. And what's happened to Mary's characterisation, now she's gone through having a child, widowhood, wrestling pigs, learning estate management and getting her mojo back, all JF can think to do with her is to have her go back to sniping at Edith? And of course Nazis or proto-Nazis had to be involved in Edith's editor-man's disappearance! He will either reappear as an anti-fascist hero or as Hitler's best friend...
Poor, dreary Edith, walking into an obvious blackmail trap with Pig Saviour Tenant Farmer! Are we really to believe that she cares enough about her baby to retrieve her from Switzerland, but then to stash her locally, and plan to claim her solely in the event that Editor-man returns alive and willing to marry her? In which case, presumably they have to pretend she's their ward anyway...?