My guess is that JM was an opportunist, possibly when she was told by police Jeremy was going down and she could be going with him she switched loyalties in the wink of an eye. Conversations with Jeremy could easily be manipulated to have other meanings in such a way that Julie quickly became to believe herself that he had done it as that was her best bet. The NOTW offered a nice cheque, very enticing and a photo in the paper may have brought forth another opportunity, it didn't but it was worth a try and the £25, 000 gave her the opportunity to move to Canada which offered her endless opportunities.
Maggie, it strikes me that there is greater residual anger about the way Julie walked away, Scot free, pockets well lined, to a new and lucrative future, having, supposedly, been aware for more than a year, of Jeremy's plans, than there was about Maxine Carr, who wasn't there but gave Huntley an alibi, presumably because his hand in the horrific crime wouldn't cross her mind, Whether she deserved it or not, SHE served a prison sentence for what, IMO, was a lesser crime.