You make an excellent point, as always. If MK was indeed coached to tell a story that would qualify for the Rachel Alert, I'm sure the police have learned the truth by now. I still recall the press conference where Chief Dinse sang MK's praises as a witness, saying something to the effect that they had gotten "a
tremendous amount of information" from her and that the eventual solving of the case would be to her credit. Obviously, what has been shared with the public in no way adds up to "a tremendous amount of information." I think it's likely that the police gained a great deal of information from MK (bless her heart!) but have chosen to keep this private. In short, I think that the public claims that MK's story has remained unchanged throughout this ordeal is disinformation, intended to keep the suspect(s) guessing about what the police really know about that night.
FWIW, I have serious doubts about the validity of the "armed gunman" story, as it has been presented to the public. However, I do believe that the family (and apparently the police) have reason to believe that whoever Elizabeth left with represents a genuine danger to her wellbeing.