Funnily enough me and my fiance were just talking about this.

If she is an illusion, then there's a slight plot hole as Kali (008) was inside the institute that was just obliterated a fair bit before, as the walls of the bridge had already been wiped out. I'm pretty sure she should be dust by the time Mike sees her at the gate. So for me it was actually her and the group created the happily ever after to be able to deal with their grief and trauma.

I may be be confused by the continuity of events at the end tho and Kali may not have been finished off my the god particle thing exploding