Actually, Karen mentions that she has used John's Market (the market that Doob worked for when he delivered groceries to the McCanns) before.
Also, when Julie opens the door to Doob (heard over the phone with Karen) she says "Hi!" in a way that indicates she knows who he is.
Doob seems to scout out his targets at least briefly before attacking. He does this with his second victim too, the one Karen tries to warn.
I'd say this cloud is Cumulo Nimbus.
Didn't he discover America?
Penfold, shush.

I think it was probably meant to make fun of Julie, just to anger the parents even further. However, I also think it's a plot hole. As far as we know, Julie and Doob had no prior contact with each other, and the 2 minutes or so it took to rape and murder Julie wasn't enough time for him to know that she had a stutter, especially when all she was doing was screaming and fighting for her life.

And if you watch closely, just before he says the line, he briefly glances into the camera, like "Listen to this, folks."

Yeah. It was basically to taunt Karen since he just got away with raping and murdering her daughter, as well as, shows that he had no remorse for what he had done. Also it leaves no doubt in Karen's mind that he was the guy responsible since he knew about Julie's stutter.
"You ARE NOT a god!"
"Why don't you pull the trigger and find out?"