which is why I said that rule 2 would have a very explicit explanation of what does and doesn't constitute "inciting violence," which would essentially be just a summary of US law.

But what you said was stupid. You can get targeted for perfectly legal speech. In fact, most advocacy for violence and even threats do not violate US law. They are a big problem for any forum hosting them though.

Because the entire complaint around it is that currently it is vague and applied too widely.

It seems pretty straightforward to me. What precisely is it applied to that you think it should not be applied to?

Unless you really believe that the example here would get someone arrested and convicted under the current US speech laws? Because I really, really don't think that would happen even as far gone as the justice system is.

Definitely not. But if you haven't paid attention, this is what gets forums targeted by governments, registrars, etc. Particularly if you allow this sort of thing and then one idiot who posts here then proceeds to put that into effect.

What's the "constraint" that requires rules 15, 16, and the sharing manifestos part of 1?

The New Zealand shooter manifesto got numerous places shut down.

As for 15 and 16, see what happened to Consume Product.