Our S&W's have all had mag safeties since 1991 or so, M&P included. We don't have the key lock.
On the mag safety- over the last 19 years, I've seen nigh onto 2 million rounds go downrange from S&W pistols with the mag safety. The only failures to fire I'm aware of were caused by improper magazine insertion and most took place on the 2nd round fired. This is exactly what would have happened in a weapon without a mag safety and the required remedial action-tap, rack, ready-is exactly the same.
The guys who complain about not being able to fire the round in the chamber aren't thinking things through. The only time this would be an issue would be in a tactical reload. You do this from cover either during a lull in the action or after everything's over.
Payne, take a real close look at the Streamlight TLR series. Make sure you get the 1913 rail key, the M&P rail is 1913 Picatinny spec.