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".
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.