I had some basic problems with the episode. First, as soon as Tess/Nathan showed up I knew they weren't real; the memory-sharing thing was too obvious a cause. It didn't end up being a big deal; the actors really sold it.
Second, since when is Jo so dumb that she wouldn't pick up on real-Zane's confusion and mistake him for fake-Zane? That was just poorly written. The confession was great, I'd just have preferred real-Zane to overhear her telling fake-Zane.
Similarly, Carter's issue should have been the intelligence thing. I really thought they were going there, and there were a lot of signs pointing to it - Stark's badgering about vocabulary, Allison liking the science talk, and so on. That was, in my opinion, a giant dropped ball.
Finally, Grant is willing to trust a shadowy organization not to steal a weapon? He had no way of knowing they would build a Bridge Device. Even then, can they really be trusted with it? Another aspect of people being dumb for the sake of story.
For all that it was awesome seeing some old players again, it really felt to me like a lot of this episode was artificially designed to get certain cast members for/against sending Grant back.