And note that Western liberalism/progressivism has pretty much created the first culture in history with strong norms against torture

I agree that Western civilization has many unique accomplishments, I would argue that it is therefore worth defending.

(extending to things like child discipline). It's inconsistent and hypocritical in applying those norms to itself, true (especially regarding imprisonment)

I'd argue that these are examples of taking the prohibition too far. In any case if Western civilization collapses because parents failed to adequately pass it on to their children, or because it is no longer capable of dealing with crime (for example), its replacement will likely have a lot fewer prohibitions on torture, and probably no free speech or free inquiry, nor anything resembling democracy.

and wouldn't it be a very convenient thing for a "rational", non-squeamish social engineer to optimize away? And then where would the slippery slope end?

This is actually my biggest issue with "progressives", you destroy traditional Schelling points on the grounds that they're arbitrary and "irrational" and then discover you have no way of taking non-extreme positions.