Surely your first point revolves round whether or not your wife believed you capable of murder?
It very possible he told her that they'd always be together and they'd have a wonderful life.
But he can't implicate her now without putting himself centre stage.
Many people who believed Jeremy guilty also at the same time had a lingering belief that Shelia could have done it. We have to remember Shelia had mental health difficulties. In many a sense he had a very convinient scapegoat.
The relatives BW, at the beginning it was all " ohhh I'm sorry Jeremy, holding his hands hugging etc, it's only after they began to get suspicious
For Julie however she must have known from the outset he had done it, he had told her of his intention and had been planning it for a long time, he had phoned her.