The thermal paste itself may not have to be cleaned off since it's nonconductive, and the contacts will pierce through any film. But arcing may have created carbon deposits, which should be cleaned off since they don't conduct as well as metal but do conduct enough to create shorts between pins.
The VRU for the CPU is designed to put out more than enough current to cause arcing and smoke, so it may have survived the incident.
I've never plugged in a CPU backwards, only 28-pin ROM chips, and most survived, despite getting hot.