To keep from energizing high speed when the other switch is in the park position, you could supply the high speed switch from the low speed switch output. In other words, the other side of the low speed switch that supplies the low relay power. One thing your forgetting is that you have no motor "brake". The park position stops the motor very fast by shorting the both low input and ground leads together. That is why there is a ground position on the park switch, otherwise the motor will coast to a stop.