Firstly Kate wasn't offered a deal. She was simply told that if she admitted that her daughter had died in an accident she would serve less time than if she was found guilty of murder. That was simply the truth. Even her lawyer at the time admitted that the 'deal' had been a misunderstanding.
Secondly the main protagonist's statements made it to the files and not one single question to any one of them whether they knew each other. Why do you think that was?
The "misunderstanding" wasn't reported in the available files, was it?
The McCanns were not officially questioned between 10th May & 5th September & there is no record of what discussions took place between them & the PJ during that period. RM made 3 statements during that period yet the question of familiarity doesn't appear to have been asked by the PJ - despite the fact he was under suspicion of abduction. Wasn't that remiss, if motive was part of the process, or were the PJ working only on the theory of collusion by investigating his movements & contacts?