So water hitting your car causes risk.

Yes, it certainly does. A sudden splash that opaques your windshield and/or distorts your view -- which a water balloon can most definitely do -- can startle and disorient the driver, leading to dire consequences. Throwing water balloons at vehicles is not a harmless prank. It is a thoughtless act that can directly endanger others. It is shortsighted and naive to characterize it any other way.

I guess you never drive when rain is predicted.

First of all, rain is an act of nature. Consequently you're going to have trouble equating it to voluntary human action. Secondly, rain can indeed endanger drivers. As can sudden splashes from puddles, clouds of spray from passing trucks and so on. Mitigating these risks is a big part of why cars have windshield wipers, why companies sell products like "Rain-X", and why sane people drive differently when rain interferes with their view of the road and/or the vehicles on it. And yes, absolutely, if rain is predicted, it is factored into my driving plans. Likewise snow, hail, high winds, or sandstorms.

I think your logic is flawed.

You have not demonstrated this in any way. You may, of course, continue to attempt to try. :)

So I'm trying to identify the edges, if any.

There aren't any when offense is the metric. There certainly are where incitement is concerned, though. You put your finger right on one of them: when incitement is directed at someone who is not competent to take responsibility for their own actions -- such as your putative mentally ill minor.

Offense is real, and measurable. It can be measured with medical tools, like you can see a bruise on someone's nose when you hit them. Yet the nose is sacred to you, and the ears aren't.

Pleasure is real and measurable with medical tools as well. So is itching, the length of your fingernails, and the salinity of your tear ducts. The point is that measurement is not the determinant. The determinant is, is it harmful, and with offense, the answer is no unless you undertake self-caused harm entirely on your own.

For your benefit, from my other writings:

What offends you may not offend me. And vice-versa. What serves no purpose for you, may serve a purpose for me. Be it intended offense, or otherwise, or both at once.

No one in the USA (or anywhere with sane law) has the "right to not be offended." Being offended is subjective. It has everything to do with you as an individual, or as part of a particular group; it varies due to your moral conditioning, your religious beliefs, your upbringing, your education; what offends one person or group (of any size) may not offend another, nor a person of another grouping; and in the final analysis, it is also flawed in that it requires one person to attempt to read the mind of other persons they do not know in order to anticipate whether a specific action will cause offense in the mind of another.

And no, codifying an action in law is not in any way sufficient... it is well established that not even lawyers can know the law well enough to anticipate what is legal, and what is not -- any more than you can guess what is offensive to me, or not.

Sane law relies on the basic idea that we try not to risk or cause harm to the bodies, finances and reputations of others without them consenting and being aware of the risks. It does not rely on the idea that we "must not cause offense." It relies on the idea that we must not cause harm.

Law that bans something based upon the idea that some individual or group simply finds the behavior objectionable is the very worst kind of law, utterly devoid of consideration of others, while absolutely permeated in self-indulgence.

Is being offended a harm? If so, should it be illegal?

No. And no.

That would move the discussion from "offense" to "harm" so that a rational discussion could be had.

No, it simply miscasts offense in an attempt to gain an unjustified rhetorical handle on it.

Offense is like a water balloon thrown from a bridge at a passing car.

No. It isn't. When someone messes with your wallet, your person, your reputation, your family in like manner, or your property, you have been interfered with. Actions designed to remedy the interference can now be put on the table. This is a very sane way to look at these matters, and it is generally what anyone seriously considering them will come up with. From it, we derive that any such action constitutes violation of the principle that "your right to swing your fist ends at anyone else's nose", or in other words such action is "swinging one's fist where it impacts the other person's nose" and so we don't accept it as valid action when considering these issues.

The water balloon constitutes physical interference with your property, your path, and your ability to drive in a safe manner, thereby additionally and (further) irresponsibly constituting risk to yet others via potential secondary and tertiary effects. Your suggestion is not the same, or even similar to, someone cracking a dick joke.

Does it matter if the "offensive language" is an adult trying to talk a mentally ill minor into suicide?

This is not "offense." This is incitement and inappropriate exercise of power. You are moving the goalposts quite a distance here.

"Offense" is seeing or hearing something that you don't approve of, and undertaking, at a minimum, a line of thought that criticizes that thing. Offense may often further extend to a verbal or written reaction, but its root remains in your own thought processes, for which you, and only you, are responsible.

If others are made responsible for what you think, then it follows that everyone is responsible for everything everyone else thinks in an unending causative loop, and as virtually everything offends someone, life would be constrained to living in an isolated environment so no one could see, hear or otherwise be exposed to you -- as your speech, appearance, actions, or even your very presence might very well be offensive to them.

It is perfectly legitimate to argue that something you find offensive, should also be found offensive by others; even so, no one has the obligation to agree with you, or even to pay you any attention. But when you impose control of others based on your perception of offense -- legislation, rule making -- now we're back to behavior that can only be valid on property you own or otherwise control (rent, have custody of, etc.)

On the other hand, tolerance, that thing we offhandedly characterize as "live and let live", is a social practice that leads to people generally not interfering with others, or even confronting them with argument. Highly recommended. It's very respectful of the personal agency of others. Note that this is not advice for discussion; it simply applies to venues and happenings that you were not involved with in the first place. There's nothing wrong with amiable discussion of issues that concern you, of course, as long as the other party(s) wish to engage.

SOP in the USA. SWAT is used overwhelmingly in cases they have no business being used in. A tiny, tiny minority of their deployments are actually for circumstances the teams were put together to confront (hostage & active shooter).

Not only is it funny (with overtones of pitiful), it gets a rise out just about everyone who cares to erect an objection. Rigid, upright individuals, blood flowing copiously to their heads, cocking their virtual pistols and ready to shoot the first time someone rubs them in a manner that provides enough friction. It's a penetrating form of humor, a kind of humor that some have to stretch to get, especially those who are anally retentive. For others, it's just plugging along as usual, strapping on the first thing they come to, and then using it to probe everyone within reach. I don't know why it's got you so inflamed. Me, I'm having a ball sacking the opposition. I can't do it all the time (I'm old) but I find it satisfactory to work in spurts. And while my youth is gone, at least I can remember it as not so much checkered, as spattered. Because the rubber didn't always meet the rode, y'see.

There is no right to create a hostile working environment for women.

What you want is an environment that is hostile to men. Offended by this project? WTF are you doing nosing around the project? Offended by strippers? WTF are you doing nosing around strip clubs and the like? Offended by foul language? Why are you listening? Offended by... well, you get the idea. You don't like something, don't pay money for it, don't support it, don't publicize it, don't bother with it, etc. Find something you DO support and do something you find to be positive. Otherwise, yes, you're going to be offended, and it's your own stupid fault.

Until someone messes with your wallet, your person, your reputation, your family in like manner, or your property, your right to exert control ends on property you have control of (which usually means you own or rent it.) Other than that, you can say anything you want, anywhere you want but on property others rent or own where you are not, and should not be, in control, and sane people will roundly ignore you.

Because there truly is not, and should not be, any right "not to be offended." Pull up your big girl panties and buck the heck up. The world is not made of sugar and spice, and every effort you undertake to make it so is a Very Bad Idea.

Push your controlling ideas too far, and someone will eventually push back. Odds are you really won't enjoy it.

There is no right "not to be offended."

No such right should exist.

No one should agitate for such a right.

End of story.

"now"?

You mean all those corporate tax cuts which were handed to them which was supposed to trickle down to the rest of us?

Trickle down economics carefully explained

I am in awe of this troll, struggling forward in the face of overwhelming stupidity. Truly a toll-tastic miracle.
*golf clap*