DC Series Motor Starting Direction
I can not envision a way for a series motor to fail in a way that would cause reverse operation. I suppose it could happen if the brush position is shifted or if one brush of a 4-pole motor is not making contact. A series motor is not a good choice to drive a propellor. The speed can be reduced by inserting series resistance. The resistors would need to be sized to dissipate a lot of power. An electronic speed controller would be better. The speed controller would need to be something like a fork truck controller. How much current does the motor draw at full speed?