Unlike all other theories from supporters, this is possible.
Julie's detailed WS may be correct. Except she omitted she ever encouraged Bamber. Instead her WS states such talk from Bamber upset her.
This explains the accuracy & volume of Julie's WS. Bamber can never say Julie encouraged him.
There is a slim chance Julie may have encouraged him during her 18 months with Bamber. It was probably more of a sympathetic ear when he was complaining about the raw deal he thought he had.
Julie may have sometimes been non responsive when he mentioned his massacre plans. Believing he wasn't serious.
If Julie dismissed Bamber's plans, that may have motivated him more. To prove her wrong.
It is doubtful Julie worked closely with Bamber & had to do much persuading. If she had wanted money she would have accepted June's offer of a free house.