-Walker's purpose was quite clear. Like Nazir said, he became a loose end when things went a different direction. If you remember Nazir needed a "ghost" to do the job, (which eventually didn't pan out because of the discovery that Walker is still alive) and i think they also needed Walker's faked death as another motivation for Brody. Another thing, is that he can freely wander around Washington without arousing much suspicion (as opposed to a foreigner or something). After all, Walker is "anyone" like you say.
-It is entirely up to you if you believe their actions can be justified. It's a matter of opinion. As for the characters, it's what they believe in.
-Carrie is constructing a plausible idea, like she's been doing the whole season. It didn't say if she's entirely sure of it.
-That i can't answer. Next season perhaps.
-Brody is actually reluctant to run for congress, until it became part of the plan. As for Dexter, i must agree Season 6 is my least favorite season.
Can i just praise the young actress who played Dana? I think she also did a great job. Kudos!