I'm a lefty too, and love the M&P platform. Mine does not have either the magazine safety (it WILL fire a chambered round with no magazine) or the external thumb safety. It does have the previously mentioned "drop" safety and the typical trigger safety (trigger is hinged - bottom half must be pulled into alignment with upper half before trigger will depress and all the gun to go boom). I'm still not a fan of the trigger safety, but understand the need for one.
A couple of random thoughts for you... #1 - The "wingnut" attached to the "boom switch" is the ultimate safety for any gun (you, the shooter, with good training ultimately creates a safe shooting environment). #2 - I own 1 handgun (for now
) and opted against a DA/SA to shorten my proficiency time. I like the idea of consistency from 1 trigger pull to the next. Plenty of others on this forum will outshoot me with their DA/SA - it's obviously a trustworthy option with the appropriate amount of training. If you can commit to that training and prefer that platform - go for it. To choose that platform because you think it's a "safer" option???? See #1.
#3 - As a lefty, I initially switched the magazine release to the other side of the M&P (VERY easily done in about 1-minute). However, I switched it back with the thinking that most guns I will shoot in the future (purchased, borrowed, or "acquired" in a time of need) will be set up for a RH shooter. So, I'm keeping it simple.
Best wishes to you regardless of your final choice.