Didn't her phone go off grid immediately after she spoke to the boyfriend? Which would have been a good five or ten minutes before she was seen on CCTV with WC? I can't imagine that she ran out of battery a short while before she met her killer. I would think that she switched the phone off after the call. Why? Not sure, as she still had a little way to go and her headphones suggest that she used her phone to listen to music. Perhaps she thought it wasn't safe to be on the phone or listening to music so put her phone away. TBH if I were in that situation, I would not have switched it off, just stowed it in a pocket or bag.
There is lots of speculation that he may have badged her/told her she was in danger, so she got into his car willingly. The only people that can confirm that are SE (who sadly can't) and WC, who refuses to say anything. If that was his gambit, then it would have bought him a bit of time to drive, before she thought something odd was going on. The fact is, that she got into his car within three minutes. The amount of traffic passing would suggest that him rendering her unconscious, on a red route, with his hazards on would be very risky.