Ooh that's interesting.
My gut says it's Tobert
I do love @Tosk's idea that there isn't actually a murder (it's possible the poisoning was accidental as well I guess)
They filmed a scene with Mabel at the Arconium in a wedding dress, so am thinking that Tobert survives and marries Mabel but is Season 4's victim
		 
				 
 
		 
 
		 
 
		 
 
		 
 
		 
 
		 
 
		 
 
		
 
 
		
 
 
		 
 
		 
 
		 
 
		 
 
		 
 
		 
 
		