And to add to that, what are the chances that Yoren could keep JH imprisoned? Even Arya "saving" him and Biter/Rorge from the fire suggests to me that he MAY have had other options and wanted to have Arya have him in her debt. What I'm saying is that JH in the black cells, then being sent to the NW was contrived.
Tywin raises a really interesting question though. If FM are sent to kill someone, and JH fulfilled his mission, whom did he kill? I'd love to think it was Robert, but Cersei is pretty much on the hook for that given the circumstances.
Tywin, can you make a case for Robert, or anyone else?
I think the case for him killing Robert is the circumstances of it happening. Adding some extra "pow!" to the wine seems to fit the pattern of all the other FM killings that we've seen. Cersei is the obvious culprit, but obvious isn't GRRM's way. Also the Lannisters (Cersei included) were riding high and succeeding. Killing Robert was not a smart move for Cersei, even with the threat of Eddard telling Robert about her.
As for FM not being able to be jailed, only needing to be let out of chains in a fire for the sake of striking up a conversation with Arya, etc, etc... We build these guys up too much. At the rate their prowess is growing on our forum they should just kill people with their laser-beam eyes.