OK, on New Alexandria, Kat gets shot in the head. Noble team brings her with them. There's is no proof that Kat died. Instead, she suffered brain damage leading to memory loss. Due to this, her voice/accent was changed, because she had no recollection of her previous identity. She was taken to Earth and when she woke up, she had know idea who she was and adopted a new identity, Miranda Keyes. Halo 2, on Cairo station, during the award ceremony, Hood gave Miranda a medal for her father Jacob Keyes' heroism on Halo. The medal was ACTUALLY for Kat's bravery on Reach, but since she believed she was Miranda now, Hood didn't want to upset her and played along. Halo 3, Miranda crashed into the Ark's control room to stop Truth. In Reach, Kat was also a bad driver and crashed all the time. She also hesitated when Johnson told her to kill him and then herself. The reason was that subconsciously Miranda remembered that, as Kat, she used to teamkill Noble Six all the time, by running him over or driving him off a cliff. Because of this, she was hesitant to TK again. Also, after Truth shot her, Johnson carried her body to a Pelican. Again, we don't find out if she actually died. This parallels how Noble team brought Kat's body along with them to be evacuated. That's my analysis. It all makes sense now.