That's a great writeup, GreenDevil - thank you.
With regards to SS potentially getting into a vehicle, I'm just trying to put myself in that situation to come up with some ideas...
My first thought was that she may have been concerned that she would be waiting there a long time (as Bart said), and so was open to options of getting home ASAP. Along that same vein, getting into a car would possibly feel like the safer option than standing on the street by yourself so late at night. At that time, no one knew that there was a killer on the streets, but it would still be reasonable to assume that a young woman would feel a little vulnerable in that situation. She didn't know that a taxi was only a couple of minutes away; we know that now, but she may have just wanted to begin her journey home. Another hypothetical: if she had had a big day (Australia Day) and happened across what seemed like a safe and free trip home, perhaps that was appealing. She may have come from a wealthy family, but that doesn't mean she wasn't watching what she spent or saving her money, or something along those lines.
My breakdown of "voluntarily got into car" theories:
1). Taxi (real or not), thinking it was the one she had ordered.
2). An acquaintance (as GreenDevil said, not anyone of special significance, but perhaps a somewhat familiar face)
3). A police officer
4). A friend or family member
5). Different service worker (e.g. Telecom)
6). Random member of public
1). Possible, but would be quite risky and I would think a fairly high chance of witnesses. Add on thought to this one - not a marked taxi but a friendly looking driver who flashes something of an ID and says he's a taxi driver. I would think this unlikely as it would arouse some suspicion and I don't think he would have a 100% strike rate with this approach.
2). Possible; a familiar face may have been welcomed at 2am on the side of the road. No perceived risk. Free lift. Home ASAP. However, this opens up questions of (un)lucky timing/stalking/possibility of her refusing etc. It could be that a murder was planned, but this particular pick up was opportunistic, but this doesn't sit especially well with me in terms of criminal "serial killer" psychology. The more I think about this theory, the more questions are raised. A bouncer fits in here also, but again - they could be easily refused; I wouldn't anticipate a 100% strike rate.
3). I would think this unlikely unless she was groomed prior (e.g. chatting to an off-duty cop in a club; accepting a lift would be more likely, having spoken to him previously [and harmlessly], and with that knowledge of his background). Another option that, IMO, is very unlikely to have a 100% strike rate.
4). As previously said, I would hope that any potential friend/close co-worker connection was thoroughly investigated. I would highly, highly doubt family given his/their other victims.
5). IMO, this is unlikely. A random telecom/local service worker would have to be incredibly charming to not arouse suspicion; definitely would not expect a 100% strike rate, regardless of charm. Their vehicle, if marked, would also be potentially identifiable by witnesses.
6). Still a possibility, even though we are always looking for connections and "reasons". I was just re-reading about the Moors murders after the recent news re Brady's Dr friend knowing his "secrets" and planning a tell-all. Anyway, reflecting on the MOs of Brady/Hindley, the Birnies, and other similar couplings, it is a possibility that there was a female driver/accomplice offering a seemingly harmless and friendly lift. I would think this would be one of the least threatening and the least likely to be reported as suspicious if a lift was not accepted.
With regards to Karrakatta, if this is the same offender, we are already assuming some development/evolution and a significant change in MO if we subscribe to any "voluntarily got into car" theory. However, some are more likely than others - IMO. Option 1 may be more likely as the "attack" could happen within minutes of getting into the car; different method of abduction but still using similar tactics to attack/subdue. IMO, options 2 and 4 become far less likely. As does option 3 (if any grooming happened). A killer who "grooms" is very different to a killer who opportunistically "blitzes". This leaves 5 and 6 - neither are especially strong theories in my mind, but they can't be discounted either. IMO, theories 1, 5 and 6 do fit a little better with the Karrakatta link in that they allow for that anonymity and there is the possibility of a similar attack style, albeit evolved and with a different method of abduction.
Anyway, I'm not sure if this is adding much of interest to the discussion, but I thought it might help to further flesh these out - especially with the view of what might be more likely for SS.