The show-type-column setting was added in GTK 3.24.12. You discovered yet another issue with NIX_GSETTINGS_OVERRIDES_DIR mechanism.
Since GSettings overrides only allow to override default values as a part of binary gschemas description, we need to generate the file for a whole schema in order to change just a single default value. The mechanism is intentionally impure in order to avoid having to rebuild every package if user changes some GTK default value. This means the application will see a GSettings schemas from the system installation instead of the ones added by wrapGAppsHook wrapper, even if the system ones are outdated. And since GLib is designed to crash when missing a GSettings key (gschemas are basically part of code, even if loaded externally, and it expects consistent system), crash is what you get.
I am afraid there is no solution to this other than having consistent system or forbidding impurities (at the cost of rebuilds). Well, using some kind of runtime file system overrides instead of environment variables to make impurities would work but would also be messy and hard to implement.
The best option is to avoid the overrides by writing the settings to dconf database directly but that is not implemented yet. It might still cause issues when a type of some setting changes between versions but that would break even non-NixOS systems so hopefully people are smart enough to avoid doing that.