Regarding the: "Why did NoCakeNoCode add drag_torque? To be perfectly honest, I can't think of a good physical explanation, but I can't argue with the results."
It comes to play when there is no throttle input, and minimum rpm is not clamped to idle rpm.
What would happen is that the drag torque value would keep reducing the rpm below zero since the drag torque is a non-zero (particularly the engine_brake component of it, since at the 0 RPM it is still a non-zero value), and engine torque, since it is multiplied by throttle is zero.
By adding drag torque to engine torque, it simply cancels out the drag torque later in the calculation..
(sorry for the elaborate and probably even more confusing explanation :D :D )