The Mini-14 has also been used in mass shootings: https://en.wikipedia.org/wiki/Ruger_Mini-14#Criminal_use
>Its design is not optimized for controllability when rapidly firing multiple rounds
Yes, that is a real disadvantage of the Mini-14 compared to the AR-15. And for military use, when engaging targets over 50 yards away, it is important. My understanding, though, is that most mass shootings happen at fairly close range, where the AR-15's reduced muzzle rise isn't as much of an advantage.
>This is why people whose job descriptions involve killing people at relatively close-ranges (military, SWAT) use this style of weapon, and not a weapon that looks like a Mini-14.
The military didn't choose the AR-15 for its performance at close range (like under 25 yards). At that range, the AR-15's greater difference between bore axis and line-of-sight becomes a disadvantage compared to the Mini-14.
>The addition of a pistol grip does lend increased tactical maneuverability to a rifle.
Do you have a source for that? Personally, I haven't found a pistol grip to make much of a difference in maneuverability. The overall length of the rifle certainly makes a big difference, though.