"the thing that absolutely terrifies me the most. This MP15-22 going into full auto mode".
What occurred after a Project Appleseed "instructor" worked on the gun is the only instance of a 15-22 going full auto I've ever heard of and since Project Appleseeds' ban I've looked for another instances of this.
The 15-22 uses a std AR-15 trigger group that is proven to be very reliable.
Enjoy your new 15-22. I have two I bought to shoot NSSF Rimfire Challenge and other events with. I've been shooting them for 3+ yrs now without incident.
As to Project Appleseeds' ban, my opinion is that they're playing CYA at S&Ws' expense rather than admit that putting shooters elbow to elbow on the firing line is asking for problems.