I don't disagree with your opinions as possibility and did think of them myself. My focus on passion was not the engagement ring it was more the burning. There are many a reported case of a breakup and a woman being set alight. This I feel could be a variation on that. The engagement ring going missing supported that theory.
Here are a compilation of news reports at the time including photos of Deborah, the car and areas she was seen.
Mentions "small amount of cash".
[video=youtube;UbT24_IDZt0]https://www.youtube.com/watch?v=UbT24_IDZt0[/video]
[video=youtube;EqWmzgOzD-s]https://www.youtube.com/watch?v=EqWmzgOzD-s[/video]
My main reason for leaning away from the jealous lover is the location she was left. It's. ..bogan... for want of a better word. Mundane location for crime passionale. They tend to dump somewhere more secluded. ..or more "angry" than a shopping centre carpark.