although it seems lightly implausible (but not impossible) that they infiltrated a Siberian penal colony just to kill a guy so he could be used as a martyr.

Well, they wouldn't have to, really. It's not that hard to pay a crooked prison guard these days. But yeah, my money's on a prison beyond the polar circle being bad for you, especially when you're not that healthy to begin with, as well.

Edit for the fans of conspiratorial version: his lawyer visited a couple of days prior, so maybe not even a guard, or a guard was paid not to search the lawyer to zealously.