Also if I actually do that right now I see a bunch of WebContent processes, the apps themselves and then there are a handful of daemons that have InfoPlist.strings open from WebKit (from the iOSSupport subsystem) but that's just a localization file and I don't know what it's doing there but it appears to be completely irrelevant (the daemons do not have anything else from WebKit loaded).
AFAIK macOS does not use WebKit to render UI. iOS has been known to use WebKit for text rendering in the past, but I'm not sure if it even still does that, and that was presumably in-process anyway. And even if it did that still wouldn't matter as far as applications' own use of web rendering engines is concerned.
Sure it does: iCal, Mail.app, iTunes (Music app), App Store, etc.
In any case, the OS itself does not rely on WebKit for UI, it just ships some apps that use WebKit.