There is a reason the camera keeps panning to other people in the diner because they are trying to imply that the assassin is already there. Create an atmosphere of tension for the viewer. Basic storytelling here guys. Also Carmella and AJ would not get the business...they don't operate that way. No civilians.
- They show the guy in the hat...could it be him (Bobby's killers wore hats)
- They show the baseball coach, nah can't be him. He got a little league team with him.
- They show the couple enjoying themselves...nah.
- AJ and his shadow walk in...could it be AJ's guard? Maybe. The guard going to the bathroom could be a signal for the killers to pop Tony now.
-Could it be the two black dudes that walked in before Meadow? Maybe. They hired black dudes to kill Tony before.
IMO the killer was already there, it didn't matter what Meadow did or didn't do. That was just to throw you off of what was about to happen. What I saw was Meadow walking in just in time to see her dad's brains splatter. Then you got the fade to black convo wit Bobby and Tony. At least thats what I got out of it and yea thats my interpretation.