Sounds like killing two birds with one stone, to me.
JM must have suffered a certain amount of confusion at the time. Her being pressed to give evidence which was then twisted beyond all recognition by the law enforcers because this is what they do purposely to get you confused. Then the influence ( she was only 21 don't forget ! ) and to be influenced by a team of police she'd have thought they must be right in what they say,they're the law.
After 32 interviews she'd have come more accustomed to the ways of the law,but she was also in the situation of abiding by whatever they said and had felt OBLIGED to given the reward at the end of it all because as I've said before that only for USING JM,EP had nothing else to go on. EP did use her there's no doubt about that.
To make the situation feel more authentic they,EP,even offered JM a " safe-house " which is only done in exceptionally dangerous circumstances when a person's life is at risk. It all added to their carefully thought of plan !