The answer lies in perspective, context, and perception. Aikido's highest goal is to blend with the attack, neutralize its intent, and permit no harm to either participant. As with any skill, the execution is dependent on the talent of the adherent, the veracity of the assault, and the unique set of circumstances at the moment of the encounter. In demonstrations, we should always espouse the highest ideals; but theory [ideals] and reality are two ends of a continuum that must be explored and understood.
If the attack is sudden and life-threatening, mere survival overrides any ethical prerogative and any successful application is warranted. That said, let me try to address your specific questions…
1. Why is it that many Aikido techniques could not be done to an untrained attacker, without causing them grievous harm?
It depends on your definition of grievous. In other words, everything you know is relative to someone else's perception. Will the attacker die on the spot? Not likely. Will they be permanently crippled? Probably not. What if they only suffer a separated shoulder, or a broken arm? Does that mean you fell short of your ideals? Who is attacking you and why aren't they trained? Did you prevent grievous injury to yourself or anyone else? A laudable accomplishment in any courtroom. Did you impart a life-altering lesson? An even grander accomplishment!
2. What techniques would you deem the safest to use in a violent situation without the collusion or training of the attackers as a pre-requisite? For instance, when grabbed or hit by a developmentally challenged but angry, strong and dangerous child at a high school visit, etc.
While in college, I found myself employed as a "bouncer" at several nightclubs, despite my small stature. Interestingly, most of the hooligans I escorted out of these establishments required no force whatsoever. Maybe it was the way I carried myself? Maybe it was the way I addressed them? On the few occasions that force was necessary, nikkyo, sankyo, yonkyo, or a variation sufficed; I did have to choke out one guy to prevent him from injuring another patron in the parking lot, but that was an extreme. Never did I have to resort to anything really drastic. If you ever have to do such a thing, be sure to talk to them, reassure them that it only hurts if you resist. The ikkyo series are the safest techniques in this regard.
2. What techniques would seem to be dangerous when done outside of the dojo without a trained falling partner, and if so how do they not conflict with the above trademarks of Aikido. For instance, would you do kaitennage to an angry loved one that had no knowledge of forward falls.
Just because uncle Fred gets rowdy at your mom's the New Year's party doesn't make it okay to toss him through the bay window. Techniques are like tools in your toolbox. Select and use them appropriately. Assuming you don't live in a double-wide in Arkansas, if uncle Fred breaks out his semi-automatic rifle, maybe a well-placed side-kick [yoko geri] through the bay window is okay. Otherwise, a subtle variation on nikkyo (that looks like you and uncle Fred are just arm-in arm) facilitates a nice walk outside or a chat on the couch. Just be sure to remind him that it only hurts if he moves.