The sanctions did not require the USA invasion which has been so disastrous.

Saddam didn't seem to be amenable to complying with them without serious action.

If it was such a good idea, why did it take the patriotic fervor of 9/11 and a case about WMDs based on lies and exaggerations to convince the USA to invade Iraq? Because it was a predictably bad idea which a lot of people were skeptical of.

I'm hardly denying that there were concerns.

I don't know what to say to this but to repeat myself: he was reasoning correctly about the consequences of it not being a bluff, and whether a rational self-interested USA would want to do it.

I don't know what definition of "rationality" you are using, that it is correct to trust one's life to others following it.

To call this wrong is itself a post hoc argument from hindsight that he should have foreseen that the USA was irrational and self-sabotaging and acted accordingly, and voluntarily topple his regime & empower Iran solely on the odds of that.

It's hindsight only in the most broad sense, and all empirical knowledge is based on hindsight in the most broad sense. And the literal reading of that sentence is that "Saddam" is the subject of "topple his regime". Who is saying that Saddam should have toppled his own regime?

And is this 'quite strong evidence' neutralized by recent events in Syria? What's the proper reference class here?

"Recent events in Syria"? You'll have to be more specific. And you seem to be trying to slide from a discussion of the case itself to discussion of whether the case is the proper reference class.