You start out by saying there is no agreement about what antifa is, and then immediately launch into you own definition.

Yes, which is why I said "as a general matter." There isn't agreement about where the contours of Antifa are - but that doesn't mean there isn't a general set of principles that for the most part make up Antifa as a movement, as opposed to something else. For example, there's lots of disagreements about what exactly makes up Libertarianism - but that doesn't mean that Bernie Sanders or Donald Trump are Libertarians. As for this specific one:

Part of your definition - "Antifa" involves a heavy distrust of state institutions (to the point where it is either a form of anarchism or something close to it)" is absolutely wrong IMO. My understanding of antifa is that it wants to preserve state institutions and protect them against the assault of fascists.

I think that's wrong. Antifa is against fascism, but that doesn't mean they are in favor of existing political institutions. The movement has its origins in the German far far left, and are far more anarchistic than they are supportive of existing institutions. In particular they tend to be anti-capitalist or communist, so they're generally not in favor of preserving the status quo. There are plenty of political groups that are what you describe, but Antifa is not one of them.