Remember the suggestion is for the Spy to see 'pm chats', which I took to read as the action PMs (which comes with it's own set of pros and cons). I mean it makes more sense for the spy to see a chat, but I agree this really has the potential to limit the essence of Forum Mafia if the threat of a spy around stops codes and strategy discussion. If the Spy can see the just the action... there could be room for tweaking to make a viable role for testing.

Now picking one person and seeing all actions their faction takes is OP as has been hinted at before (if you target a Mafia and see Mafia actions you have a confirmed scum right there, as well as lots of possible evidence to back yourself up i.e. you also know you was roleblocked/silenced/drugged/investigated). Having a random Mafia team assigned to you at the beginning could be interesting, I think the Spy becomes a more balanced power role that could uncover some key information (hmm person A was roleblocked night 1 but hasn't claimed it, I'll keep that in mind...). If that's considered too powerful, since seeing the actions of a whole Mafia team throughout the game could be amazing in some setups, then possibly just a single random scum each night is watched anonymously, or perhaps you see the targets of all the Mafia team but you don't see what particular power (kill/block/enforced/etc.) is attributed to which target.

Some food for thought, I think it gives the Town a new set of information which isn't powerful by itself but a cunning Spy able to corroborate his data with other Townies can shed new light and evidence.