Lemme clear a couple things up:
1) Nishi is definitely a Gantzer. He had 90 points when he died, in a mission. Like Cherry and Katou, he's been revived to be part of the team, and has to start over. What prompted Izumi to regain his memory is still a mystery, but it's pretty obvious from Tae's dreams that Gantz is using "lazy deletion" (a computer term meaning part of a hard drive is simply marked as being empty, even though the data still exists in the deleted part)
2) When Kikuchi and Akira called, Kurono's phone said "Unknown caller". It irked me that I forgot to include that, so I'm doing it now
3) There's no indication that anything significant has happened in the world of Gantz since the Oni mission. Nishi, knowing he's now on a completely different team, ran into Izumi and wanted to know what kind of people he's going to have to fight with. Izumi, as someone said, believes that (with the likely exception of Kaze), they're mostly helpless without Kurono to lead them. Therefore, he says that unless they get better weapons (e.g. they need more people to get 100 points and pick the powerful weapon), Japan is fucked in this whole big Gantz game, whatever that may be.
4) Yes, Kikuchi is a douche. He dug too deep in the wrong direction and had to sell everyone out to save his own ass.
5) It seems like Akira might not be completely evil with the recent moral quandries he's had to face lately. Thinking back, there's never really been a scene where he's shown any hatred or animosity towards Kei, even though he prefers not to associate with him.