No non-town players really recognized what a threat Derf was until it was too late. I heavily abused his role power in order to sway things into Town's favor, that and the fact that no one was really recognizing what kind of power he had, not even Derf himself. I'm pretty sure that during the game, I was the only one that knew that Derf was the key to the whole thing. The Death Seer role was unimaginably powerful, and was simply moreso when connected with the Traveler role.
I was aware when I got the Traveler role PM, and its description, that you weren't really thinking of the role as I was thinking, Neo. In my mind, with all the information I had that was immediately useful, I was better off leading the town for as long as I could, keeping them away from the hidden perils I knew of. You, though, knew that my role's visiting power was much more powerful than I knew, and so expected the role to play out more slowly. However, being used to fast-paced games where role powers are essentially useless, I wasn't so much concerned with my powers as getting the town on the right track. So I wielded my power as best I could.
Hm. As far as mafia traitors go, it's very, very difficult to do in a balanced fashion. Thing is, the mafia are working from the mindset that they are the ones set against the town, and that their goal is domination, not defending themselves. So you have to set the traitor in such a way that they cannot easily backstab the mafia, because the mafia are not going to be prepared for it whatsoever. That wasn't much of a problem with this game, because the town eliminated the mafia fairly efficiently, but Liquid didn't do a damn thing for the mafia while he was alive, despite being its third member.
The way I've typically seen it set up in the past (in a balanced fashion) is to alert the mafia of a traitor in their midst (and use anonymity or some other device to prevent them from using the information), or, alternatively, simply not tell the traitor that they are, in fact, a traitor, until some event triggers them or pulls them away. Either one of those, (the latter of which you kind of invoked), or have the traitor's goal not one incompatible with the mafia, but simply not the same goal as the mafia's.
In this particular case, I think it would have been better had the mafia known Liquid was some kind of Shadow, perhaps "helping the mafia" or something like that. At least then they would have been suspicious, prepared to look for backstabbing.
Beginning the game, Sibare acted as a very anti-town Shadow, whereas Ellipsis was more that third party you were looking for, Neo. You have to understand that people will take roles in directions you never considered - I can remember when Gammagooey utterly broke a role I'd considered to be one of the poorer town roles in a game - and should be prepared for many different situations. I generally use "random events" as a sort of GM god tool, which allow me to correct imbalances that I notice after the game has started. I never totally change the game, but if one side is really hurting because of some oversight I made, then I can at least lend them a hand, or what-have-you.