Good, I agree with exactly what you have just posted.
Now we are past this part, at this point as the leg is extending by knee extension, the thigh is being extended by hip extension we are about to plant. Has the foot dorsiflexed previous to this (the plant) and should it?
Yes. The foot should be slightly cocked at this point.
Does plantarflexion now occur as we are about to plant?
Dorsiflexion should have occured before this point. As the athlete approaches ground contact they will actually begin to plantarflex (the action of) slightly. Better sprinters will contact the ground in a neutral to slightly plantarflexed ankle position. Extreme dorsiflexion should be avoided.