First off the stepper driver that had the heatsink fall off it is very likely dead. You said so yourself by claiming that half way through a print it stopped working. This was you explaining to the group that you overheated that stepper driver to the point of failure. And no putting a heatsink back on a burnt driver doesn't magically put the smoke back in. Once you let the smoke out it is out. you know? This damage should be limited to the stepper driver UNLESS you reconnected the heatsink in such a way that you shorted pins on that stepper driver. If you tried to put that heatsink on while things were plugged in and powered on and or if the aluminum heatsink were allowed to touch rows of pins on the stepper driver then yes you might need a new mainboard. One nice thing we have going for us with the SKR boards is 5 driver sockets. You should only need 4. X, Y, Z, and E. If one of your extruder sockets is now dead then perhaps you should be using the empty and available stepper motor socket?