I agree that talking Schelling fences is usually more productive, and that it's probably not people's true rejection on abortion (norms around sexuality and fertility probably play a bigger role). Note also that unlike you, I never saw an "abortion is murder" sign in real life, and don't remember the topic ever coming up in real life.

Schelling fences probably play a bigger role for "justifiable killing" (like self-defense, the death penalty, euthanasia), where having a strong norm against killing in general discourages revenge killings (anti-abortion seem to be trying to hijack that norm to cover a case that doesn't fall under "killing" nearly as naturally). "Racism is bad" is another case where the norm is pretty valuable and useful in itself, and acknowledging that "non-bad cases of racism are not bad" would weaken it.

I stick to my guess that it's only a tiny fraction of the time

Eh, it probably depends of the reference class you're picking, and how charitable you're willing to be in interpreting people's reasons. when deconstructing a WAitW, it may be worth directing the discussion to one on Schelling Fences / norms etc., both as a way of raising the quality of the discussion, and of leaving a line of retreat.