I suspect he stopped because LS was kicking up a bit of a storm. She was drunk and distressed and people are assuming she was crying to herself.... I think it’s more likely she was shouting and causing a scene, hence why greybeard stopped. People don’t stop their car unless there is some commotion.
He talked her down , grabs her elbow and she falls or he pushed her into the front seat..... ( as seen on the cctv )
He then proceeds to ask her where she lives and he thinks he will do his usual Pervy flashing on her and get away with it. Therefore he asks her the postcode. It’s only as he drives that he starts to see a bigger opportunity as she is so smashed , she doesn’t know where she is going .
The sat nav entry was a mistake that he made as it was an opportunistic attack. Having realised the next day he tells him sister or whoever , to make any cover story more believable.
He will surely have researched this type of crime and how to dispose of a body and that is why