Suspects 3 and 4: Magnus and Horst

Magnus

 "Horst gave me this photo of his former-ahem-friend, Magnus who now resides in Berlin," Mews explained to Quigley who was busy refilling his tea cup.  "While Magnus was out of the country by the time the notes began it is possible that he could've set something up using an accomplice."

"And Magnus' motive?" Quigley asked.

"He was a jilted lover.  He was bitter that Horst rejected his plea to move with him overseas.  He could have decided to "scare" Horst into moving with him. 

Horst

"And Horst?  As a suspect?  But he is the victim here Mews.  Why on earth would Horst harass himself?"

"Stranger things have happened Quigs.  While unlikely, we must not rule him out if we are to be thorough.  It may be that he is motivated simply by narcissism.  The squeaky wheel gets the attention, right?  Or perhaps he has decided to set someone else up.  Maybe he doesn't like Mrs Snodgrass and wishes to drive her out of the building by making her look like a dogist."
 
"That would be a very passive aggressive maneuver wouldn't it Mews?"

"Yes, yes it would Quigs.  Now, more tea and more suspects.  Are you ready?" Mews asked as Quigley wearily nodded.