Sorry a bit of a long one
I just want to add something about the theory he knew the victims.
Picture it> Sorry you have to pretend you're a sick minded killer for a tick>
You're a rapist, having recently blitzed a girl you don't know in a park, taken her in your work van to a cemetery and raped her. You're happy you got away with but are afraid you will get caught if you do it the same way again. Next time you'll be more careful cos you know the victim can give something away. You're nervous but think you can come up with a better plan.
[start murderous period]
You've already killed SS in some way (for now we'll focus on JR).
So this next time you're going to try to catch up with a girl you know ~~~because that is the best way to avoid police questioning when committing a rape or worse (sarcastic tone)~~~. You're going to go late at night, somehow arrange for her to meet at a busyish club/bar, where lots of people can see you. You have to convince her to ditch her friends, get in your car (by her own choice preferably), then while she's a passenger somehow restrain, rape her and murder her. All the while avoiding all possibilities that anyone should recognise you or that she doesn't choose to go with you.
Not to mention, if you'd arranged the meet-up beforehand (before mobiles), you would want to be sure it was totally secret. You definitely would want to be certain she hasn't told a friend and also it'd be a shame if she doesn't show.
I could add many more reasons this scenario seems highly unlikely but for now I'll finish by pointing out, KK was a blitz from a park away from the main strip. It worked but the girl lived to tell the tale, which would've made him nervous. I'm with the idea either he asked them to get in or, IMO more likely, blitzed them when no one could see. MM may have just been an overseas student or something who has never heard about the footage. Or he could've just cracked a funny joke as he walked towards her while asking for a smoke. I just don't see the friendship attack as fitting into the logical progression of the rapes & murders. I hold strongly that on the nights of the murders, if he were to see someone he knew, it'd set his nerves into overdrive and he'd have avoided them like the plague!