Re: causing no (serious) harm
In most conflicts either villain or victim will get harmed. The one who harms, harms his spirit as much as the victim is physically/mentally harmed. The victim of course bears the harm directly.
But in Aikido as in most martial arts, once you have mastered it beyond the rudimentary techniques, no harm has to be delivered in order for it to retain its effectiveness.
Take for example nikkyo. A lot of us balk at getting a full force nikkyo that will snap our wrists into kingdom hell for months. But a good aikidoka can bring you down to your knees in an instant with nikkyo but without the hurt at all. Somehow, my legs and body can be controlled by just the smallest of twitches coming from my fingers and wrist especially with my balance taken completely.
I think it has everything to do with your capability and not the art though. If any of us has the power to control an attacker without harming him but chooses to harm him instead willfully because of ego, anger or impatience, then spiritually we are harmed ourselves. Because we choose to do evil instead of good.
However, most of us do not have that power (or consistently have it), thus we do the next best thing. I.e. not leaving ourselves to harm because if we did, not only do we harm ourselves spiritually (for being stupid) but definitely physically as well.
IMHO I don't think there's any art our there that gives you that complete control from the outset. It has to come with mastery of the art, even in Aikido.