I think in all scenarios, including the official one, the police have assisted Julie and covered up certain things - pretty much I take that as a given. We already know the jury were misled about her criminal record.
Jeremy could be innocent or guilty, it doesn't necessarily matter to Julie's truthfulness which. Julie just templates certain random incriminating incidents on to a narrative the police provide to her.
As you say, Julie could have been prompted to do this by something the police told her or it could be something Jeremy told her. Maybe Jeremy was fooling around with her and made up a story about a hitman because he found it entertaining or whatever?
In any of these scenarios, the crucial point is that once Julie co-operates with the police, Jeremy is then boxed-in and his legal team have no choice but to run a defence that involves refutation of everything Julie says - hence Rivlin Q.C.'s line to the jury about them having to decide which is the actor, Jeremy or Julie.
Wouldn't a jury of ordinary people, who bear in mind were under time pressures and didn't have the benefit that we have of examining this case at leisure, just assume that Julie couldn't have been lying about everything? They would then consider the fact that Jeremy was refuting everything, rather than more realistically admitting much of it but pointing to it being out of context or exaggerated, etc., and perhaps reach the conclusion that, on balance, it is more probable that Jeremy is lying. That doesn't mean Julie's evidence swayed them. They may have decided that they couldn't reach a verdict on that basis and had to look at other evidence instead.