Looks like evidence of arcing at pin 21 in your first picture. According to the datasheet, that's one of the 3-phase outputs. Hard to tell for sure, but it looks like you used way too much thermal paste. It apparently overflowed and made contact with the pins of the chip -- possibly causing the arcing. Another possible suspect is tiny bits of solder left behind during the replacement. I can't see any in the photo.
The SMD resistor at R74 acted as a fuse. It looks like it was a 1 ohm. I doubt the large capacitor was affected. When they go, the top usually splits at the scribed lines. That's what the lines are for.
Personally, I would clean all the excess paste off while looking very carefully to see if the arcing was between two pins or a pin and a trace or what. Look for physical damage to the chip that would indicate internal damage. Replace R74 (that's going to be fun -- get yourself a magnifying visor!). If you are willing to risk another $20 on this, replace the motor controller chip again. Reinstall the heatsink, but just use a couple of drops on the chip. LESS IS MORE! You want to end up with the thinnest possible layer between the chip and the heatsink; it's just supposed to fill in microscopic scratches and exclude air. It's actually a barrier to thermal conductance -- just not as much of an insulator as air. And it's certainly not supposed to be so thick it squeezes out onto the chip pins.
Then hold your breath and try again. It is possible that R74 blew fast enough to protect the rest of the components. The motor controller chip has some internal diode shunts that may have protected it. Hard to say what might have happened elsewhere, such as the microcontroller chip, but if the arc was simply between the output phases then you might get lucky on this.
