Hi,
I think that most wiriters would like to view epubs included in search folder in editor windows just like is already done with pdf files.
As with PDF, this is something that would require either native support by macOS/Windows, or a library that provides that capability that can be integrated into other software as a flexible viewer component.
Meanwhile, I would recommend Calibre, which can convert eBooks to a wide variety of formats, some of which are more suitable to general purpose usage as source material and documents—including PDF and general RTF for maximum usability in Scrivener.