A couple of thoughts to consider... it was mentioned that prior to the field goal attempt Team A substituted and the R/U held Team A because of the substitution. Per Mechanics Manual, page 53: "These procedures, which are intended to prevent the defense from being unable to match up during normal scrimmage situations, should not be used in the case of a substitution for a last-second field goal.... The defense should be prepared to respond appropriately. In no case should the U (or CJ) be standing over the ball to prevent the snap as the clock runs out."
Philosophy on whether this should be a live ball or dead ball foul for Team B substitution on potentially the last play of the half... if it's that close, let it go live and if the field goal is good, decline the penalty. Field goal is no good, 5 yard penalty and probably an un-timed down. Give the offended team the best of both options. "You just got the count of 12 after the snap" is your response. Last year a similar play happened and took a field goal off the board and after the ILS penalty from 5 yards closer, Team A missed the kick. Team B gains a huge advantage if this is called a dead ball foul. If it's early in the pre-snap and "B" has 12 in position that's one thing to call it a dead ball foul, but if you haven't called it until the snap or just after... go live ball ILS on a last second field goal attempt.
Last item... looks like Team B only has 11 players on the field... 12th man ran off as the SJ came in to double Umpire. Whistle, flag appears to come after the snap. Should have been a live ball foul... correction, should have been a no-call unless you can show 12 on the field.
Substitution mechanics, counting players, announcements... all things in our control. Don't give things away you can control. Next time you pre-game it might be smart to discuss these and other things. This isn't criticism, it's a teachable moment.