Q. Is the neural impulse generated by the stimulus that triggers the arch reflex restricted within the neurons of this circuit?

This is obvious since the person that received the initial stimulus for example the percussion on his/her kneecap perceives it meaning that the brain became conscious of the fact, the sensory fiber that first conducts the arch reflex connects with neurons of the arch reflex but it also connects with secondary sensory neurons of the spinal cord that transmit information upwards to other neurons of the brain.