1. Not be "that guy."
2. If you're hanging out with other guys, and they start to be "that guy," tell them to knock it off/leave that person alone.
3. Talk to your sons/nephews/godsons about not being "that guy."
4. Talk to your sons/nephews/godsons about enthusiastic consent.
5. Explain to your sons/nephews/godsons that the way a woman is dressed does not count as any sort of consent to be catcalled/leered at/groped.
6. If you see a woman being harassed, intervene. This doesn't necessarily mean that you pop off at the mouth and start a fight and all the other stuff you were saying that somehow turned into getting shot. This could be as simple as say, "hey buddy, leave the lady alone," OR "hey sis! Funny running into you here, wanna grab a coffee and catch up?" or "hey honey! Sorry I was late meeting you, let's hurry up and go meet Tom." Unfortunately in a lot of these situations, other men won't back off if a woman tells them no, but if they view you as someone's sister or girlfriend/wife, then they'll back off.
1-5, I feel like I've done/do all of them. I've basically turned myself into a social pariah at work to avoid it/not have to hear about it. 2-5, yes, I do/would say exactly those things to any young man or in any situation where I feel like I can influence another.
Number 6, it depends. No, not every time is it going to get physical if I get involved. But it's a real enough risk that telling a guy to "knock it off" could certainly escalate the situation to violence. I love your ideas about pretending to have a relationship to the woman, but, lord knows how that would go. I might be the last straw "now this asshole is pretending to be my brother, oh, that's it, I could take a little cat calling, but this guy is too much, time for a face full of mace". :) I say that somewhat in jest, but I'm a kind of "scary looking" guy, I'm not sure coming up to a woman who's experiencing harassment and doing anything but confronting the guy would go well. It might.
I think the problem here, even with this new (and probably better) indirect way to confront, I just don't see the roving bands of inappropriate bankers that you ladies do. I don't see it because it's not directed at me, like most people, I'm too busy living my own life. The times I see something like this and like that I described many pages back, bad men doing bad things. I spend a lot of time in NYC, and I honestly can't remember the last "catcall" I heard during the day. Now, late at night, I do remember a few, but that's drunk guys and drunk girls, a highly volatile situation to get involved in.
What I could get onboard with (and am, would be) is if a woman came up to me and said "please help me, this guy won't stop harassing me". I'd figure something out to help her. Put her in a cab, pretend to be her Dad/brother, call the cops. I'd do something. But the standard that seems to be pervasive here is "no, the woman shouldn't have to say something, you should just see it, know it's harassment, and then intervene". Well, some cases are easy and clear, most of them are not at all. The guys whistling to a girl in a really short skirt in the bar.. Harassment!! Well, maybe, until she turns around and smiles at one of them. The deli guy making a pass at the woman in yoga pants.. Harassment!! Well, yeah, but maybe they are married and this is their little "role play". Or maybe she really likes him and wants her to notice him. IDK. And putting that onus on me, to act in this very ambiguous situations where I cannot possibly know is it/isn't it wanted.. Well, that's an impossible standard to live up to.