Before the chapter revealed the real situation, I already expected this to be either a dream or (indeed) a fake Reika.
Think about it. She said 'Everyone is dead' while we obviously know that some characters are still alive. Karate Dude, Sakata-sensei... few examples. Both Izumi and Kurono dead? Yeah right. In any case it's not her way of doing things to just give up and go for a last fuck till death arrives as she's portrayed a lot more intelligent and would first confirm the status of all members. Another simple thing... why would she even return to that place anyway? If she tracked Inaba with that device, she also would have known the movement of other living members. Makes no sense, thus she's a fake.
Heh, I actually first analyzed the situation before finishing the chapter to see if I was right.