does it really help to simply stay next to my colleague, letting her do the talking while I do nothing but looking at the karen in the eye?

Yes. It really does. Makes them feel like they aren’t facing this alone.

what if, each time the karen opens her mouth I repeat ‘calm down’ ad nauseam till she either tires, shuts up or walks away?

Bad idea. That’s what we call “escalation”. You don’t want to do that.

what do you say or do to support your coworkers while they’re being verbally abused that somewhat defuses the situation?

Give them a way out. If there’s a phone on their desk, call them. If there’s a tannoy, make a call that they’re urgently needed in another room. If there’s neither, go in another room and yell their name like you need help, from where you can’t be seen.

what if I have to do this with a man and it gets physical?

Don’t start the fight. Defend yourself. And try to leave the situation as soon as you can.

I’m as guilty as the first aggressor.

Not true. You are entitled to defend yourself and escape the situation.