A fellow official asked about the legality of SC's formation. I was so focused on the substitution issue that I simply overlooked the fact that they were in a sort of "swinging gate" formation. That has no effect on the substitution issue, but it does affect mandatory numbering. In other videos, Shaw has declared similar "swinging gate" formations as not being formations that make it "obvious a scrimmage kick will be made;" therefore, they are not scrimmage kick formations, by definition. So, a team using such a formation must comply with mandatory numbering (i.e., at least 5 linemen numbered 50-79).
Uh, guess what? I don't believe SC had mandatory numbering. With the given camera view, it is very hard to see, but as the players spread out after the ball was passed back to the apparent punter, the numbers of the linemen in the cluster near their own sideline do NOT appear to be 50-79. So, even without the substitution issue, the down 'should' have been flagged for an illegal formation, anyway.
After the substitution penalty, SC used a 'normal' punt formation, so mandatory numbering was not an issue.
We have to be on our toes.