Well I don't know what else is qtwebengine_chromium so I can't say for certain. The viewer is already my default, but you're correct I typically just open books through calibre itself. This is my first time fiddling with environment variables so yes it's pretty new to me, I wouldn't consider myself a power user in any form (win10)
So does this make sense? Or the value has to be true? Or something else still flying over my head?

.