Car engine didn't shut off even though the key was removed from the ignition switch

The problem that I am having is that the other day while i was driving the cars engine lost almost all of its power and sounded like it went into an idling speed. All of the cars dashboard gauges dropped and when i went to accelerate, the car was struggling to speed up and it seemed like it wouldn't shift past first gear. I turn the car in the off position and even removed the key from the ignition but the engine wouldn't shut off! I tried starting the car back up while it was idling but nothing would engage. Next, i opened up the hood and see that the motor was shaking violently and smoke coming from the starter. this all happened within 90 seconds any over the course of this time i could hear the motor loosing its RPM's until it slowly died and lost all power.  I tried turning the car back on but apparently it drained the battery in the process.
i take it to a shop , they replace the started and the wires that somehow got burnt in the process. they say the cars ignition switch is causing a bad relay to the motor and that might be the cause of the problem. So i then take it to a dealership, they run a code test and say everything checks out. only thing it says in the report is that the car had stored codes for low voltage.
One problem i had 2 weeks ago was my automatic starter was starting my car without me pressing the button and some how sent a signal to the starter and kept it in the on position when i was driving. so that fried my last starter (it actually caught on fire) so I immediately removed that device from the car. The alternator is only a month old (previous starter also burnt that out) and the battery is fairly new.
I have asked around and no one seems to know why the motor wouldn't shut off and cause it to burn out my starter. If anyone know anything, I;d be more then happy to hear from you.