I seriously doubt Desjardins has any worries about repercussions for killing Montagna other than from the Mounties. The Bonanno's are not after him, it's the Sicilian's in Montreal that he will have to worry about. Mirarchi is his only shot at living. But him killing a made member doesn't mean shit in Montreal anymore unless the guy was a Rizzuto or Ndrangheta.
As for his sentencing, I don't know Canada's laws but I wonder if he gets out of that plea agreement since the evidence couldn't be used. I don't think I've seen a case like that before.