That's an interesting theory too. My thought was that after their encounter outside Duchess Boutique, he walked ahead of her and turned onto Hope Street as though he was going to leave her alone. She dawdled behind very slowly, speaking on her phone to allow for him to get a safe distance ahead (safe enough for her to pick up pace again.)
Your theory would explain why no-one seemed to hear/see anything unusual, even in the laneway where "Sally" the resident didn't notice anything suspicious (where the handbag was found).
I too think he had watched her before and knew her route home. I mean, how did he know she would turn onto Hope St? The other young lady in the article posted here mentioned that she had encountered a similar man twice over a matter of weeks, as though she was singled out.
The only other way he could know where Jill lived was if he had used his, "Someone's following you" spiel and she had accidentally said, "Oh, I'll be OK. I live just around the corner."
Personally, I think he'd been planning to murder for some time. The women who had encountered (what they believe) was the same offender, both said they were sure they would be killed (the backpacker and the aforementioned woman who was approached by him twice over the weeks.)