That's right.
He trusted her enough to involve her in the caravan break in. No doubt spending some of the money with Julie which may have impressed an impressionable 20 year old.
Bamber then decided to involve her from afar in the massacre. Ringing her up at 3am and then whisking her over the WHF. Probably expecting her to be impressed with the nice food and partying he would indulge her with.
However Julie wasn't too impressed when she heard him ask another woman out, was she ?
I cannot comprehend your explanation of how an ordinary 20 year old girl thinks. Why would she be 'impressed' by a boyfriend who stole from his family, surely most young women would find that frightening and unattractive?
She was a person of free choice, of average or above intelligence because she was able to gain good enough A level's to be accepted at Goldsmith's as a trainee teacher and then achieved good enough exam results to be accepted to stay on an extra year to gain her B Ed degree.
On one hand you like to claim she was Brain of Britain and on the other that she was easily impressed, naive and vulnerable.
It seems to just depend which suits your story of the moment and doesn't really prove anything.

Imo, Julie Mugford was pretty ruthless by nature and always looked after herself first.