I agree Nick remembers everything and that he and Percy are in cahoots. He's offering up too many thinly veiled references. One of them took the pictures of EJ and Abby, which shuts up both of them. When she finds out about the infidelity, and when she realizes Nick's memory is intact, Sami has ample reason to leave town, which I see her doing with the kids. Kate will be able to stick around, because she'll give Nick his desired employment, and as long as he can jerk her around, he won't spill the beans. Then there's Gabi. It wouldn't surprise me if she winds up dead at the hands of Nick. He's smart enough to get away with the very scenario that the three amigos tried to pull on him. That could give Sami even more reason to skip Salem.
I have to say, knowing all that we know about who's leaving the show, this could play out in a really interesting way. I certainly see the reason for bringing Nick back to town...much more so than for bringing back Kristen.