Am I right in understanding that the proficient Aikido practitioner can render a person unconscious *without* resorting to violence ... ?
An aikidoist can do whatever he's been trained to do with aikido or non-aikido techniques.
Generally speaking, judo is known for its "sleeper holds" in which an artery to the brain is pinched, rendering a victim unconsious until they revive or are revived. Such techniques can also lead to brain damage or death.
In aikido we use painful pins on the arms or legs. While in some cases such pins can damage the muscles and tendons, or break bones, they do not cause death.