These are interesting claims and points, perhaps, but remain disconnected from the original material. In particular:
> They can incubate them elsewhere. Which, indeed, they do, as I have said.
This needs an immense amount of development to maybe qualify as both connected-enough to this topic to belong in Scott's piece, and a strong enough claim to be worth either explaining and refuting, or adopting and defending. You're claiming that things like declining to prosecute a war against the USSR after WWII is an example of an action that acted as an outlet for what would otherwise have become domestic US leftist political violence in the US. There are, like, several things about that, and your other examples, that need to be filled in before it might be clear that makes any sense at all, plus some kind of pattern of this kind of thing increasing over time needs to be established that can't easily be explained by other, more-straightforward factors. Notably an awful lot of these examples are failures to act—what's that about? How's that an outlet for a kind of "energy" that would otherwise push the US closer to a reign of terror? Why should we think that sort of thing can act as such an outlet? What's the connection between those things? I see none whatsoever—it is not obvious this should be entertained as a relevant and strong line of inquiry.
> Also, democracies can grow more leftist over time (which, I would argue, they have) without becoming internally unstable, as long as a majority of voters continue to vote for policies that move further and further left.
Maybe! But it doesn't appear to make them ever-more violent in the specific way in question. The real events and trends we have before us really don't appear to fit—not to fail to fit your claims, but by the claims made by the folks Scott wrote the piece to address. (This isn't me arguing against you—I follow that you do get that what you're aiming at is Scott's alternative model, not that you're arguing in support of the Reactionaries)
You have proposed some reasons different from Scott's that this may be the case, and fault him for not addressing your proposed reasons, but it remains unclear to me that there's a strong line of argument there, specifically as it relates to the topic at hand. It's not clear to me that he should have brought it up, or that it makes his argument weaker that he did not, let alone that it's part of some set of "schoolboy mistakes" to have not done so.
What I don't find any of this back-and-forth convincing about, is that this piece of Scott's is in error for failing to address this stuff. I definitely am not convinced that failing to entertain (or even mention) some kind of, "the 'temperature' of US leftist domestic political violence has, perhaps, remained cool only because we did stuff like not do much to help Chiang Kai-shek" explanation, constitutes an elementary error.
