If her friends salvaged the glass she was drinking from and kept hold of it that was amazingly quick thinking, it almost seems too good to be true. The only way this could have happened was if AP was clearly acting very, very creepy. If this was the case her friends would have to smuggle the glass out as glasses or bottles are not allowed be taken out of clubs. I don't think bouncers at the club would have been much assistance to her friends.
If her friends had not kept the glass it was luck that the club hadn't washed their glasses yet and police contacted them very quickly to preserve evidence. I think her friends must have known something we don't as the police seemed to respond and take this seriously very quickly. What struck me was how quick her family flew over, the police must have told her it was a very serious situation and what they suspected. When people have been missing in Glasgow before I've seen their friends and family posting flyers within a day or so all over the city centre but there was none for Karen. They must have known very quickly something sinister had happened to her. Her mum and dads faces during the news conference looked so grim, I think they must have known more than just she went missing.
Regarding the knife, Glasgow is well known for knife crime (it has gotten much better in the last decade) so I'd say there are probably quite a few knives in that canal so it may not be linked.