And in that same thought, he could have called/text the assistant (aka W1) to tell him to be prepared to do this first thing in the am. There could have been a standing time (or prearranged sometime on Wednesday before this happened) for the assistant to come pick him up in the am. He would NOT have wanted the assistant to come to the house. Much like they sent a text to NG to stay away. I think he really did try to keep more people from getting involved.
Or SS knew that the assistant had plans that morning that didn't involve coming to pick him up, but to do other work. So he made arrangements for him to be free at ANY TIME that Thursday morning, since at that time, SS wouldn't have known exactly what time he would be able to get the cash together.
This is why I don't think it is suspicious that the assistant didn't respond to the 11:54am call...he had done what his boss told him, and then his boss gave him other tasks to complete, or he went about his regular day...not worried about SS. He had completed what his boss asked him to, and was just on about his day. I don't believe for one second that just because he missed a call from his boss after having been in touch with him that morning, that it makes him complicit.
People really do miss calls from their bosses, and call them back a couple of hours later. Especially if he had been told he wouldn't be needed the rest of the day. Which I can see SS telling him, to make sure again, that he stayed away.
I am pretty sure that unless SS left a message, we will never know why he called.
If like many of us believe, that the assistant was not involved, it is terrible the guilt he must feel (even if he shouldn't)...and the fact that he is being called a suspect could really traumatize him!!