I seem to remember A Stitch in Time leaving lots of unfilled time in Garak's backstory. It's been a long time since I read it, so I may be wrong, but I don't think the idea that he might have infiltrated the Federation at some point automatically contradicts the novel. Yeah, but there again we might get the benefit of another brilliant actor. That's not so much a "disadvantage" as it is a creative decision with both pros and cons. I mean, that's a feature, not a bug. There is a point where a creator has to acknowledge that it's questionable whether or not a mass audience of millions of people will be able to connect to a primary character wearing a rubber face as well as they would to a main character where we can see the actor's face.