Presuming the 28 point "Mercy Rule" is included in Massachusettes Pop Warner rules, restrictions placed on play after that point differential is reached, are directed at Coaching staffs regarding the type plays called and restricting certain players from participating further at certain positions. Field Officials are responsible for altering the game timing to adjust to "running" time, to shorten the contest. Once a "Running Clock" is started it does not revert to standard football timing rules, no matter what happens regarding scoring, for either team.
As is the case with NFHS rules, Field Officials are responsible for removing players showing signs of potential injury (not limited to only concussion suspicion) to their repective sidelines for evaluation. The teams bear full responsibility for having qualified medical personnel on site for determining if a player is fit to continue participating. Should a player, returning to play after being sent to the sideline for evaluation, display questionable symptoms of concussion (or other injury), field Officials are limited to sending him back to the sideline for further evaluation.
Other than a vague suggestion about an alledged dead ball injury, what are the field officials accused of? Concussion injuries are not necessarily the result of "illegal" contacts, and are far more likely the result of inadequate instruction and coaching. Field officials have little pre-emptive control, and no authority, over contacts, legal or illegal, prior to their being applied.