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DrChiper - maybe this will ring a bell ==>>
Nirsoft's Search Filter View
It allows one to 'force' an iFilter onto a file type. When I first got Win 7 (2008/9) I think I tried the iFilterShop ZIP iFilter. As I recall it sort of half-worked. but it treated the zip content as plain text and I couldn't find a way for get it to use the HTML iFilter. The end result was that the index would become hopelessly corrupt and search would stop working; I might even have gotten some BSODs because of it.
At one stage I decided the only way to fix it was to re-install windows, which I duly did but it was still broken - then I realised all I had to do was to uninstall/reinstall the Indexing - that fixed it until it broke again
Once I stopped force feeding EPUBs into the ZIP iFilter I had a stable system again.
At the time, I searched high and low for some Open Source ZIP and HTML iFilter DLLs, back then I thought might have had a go at producing a EPUB iFilter, but I never found anything,
If MS persevere with their Edge support for EPUB there maybe some hope that they might provide EPUB Support in Windows Explorer ans Search.
BTW : you may have seen me suggest that MacOS Spotlight could search within EPUBs, that turned out to be fake news. All it can do is search on HFS extended attributes - which is a bit like MS keywords.
BR