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For what it is worth, I run all my ebooks through Sigil/epubcheck (I prefer Sigil to calibre's ebook editor for most tasks) before sending them to my ereaders. I have seldom seen as many errors as you seem to be seeing and those few were from sources such as one book from archive.org where the program they are using to convert to epub seems to have been written by someone who once heard there are epub specifications but never bothered to look at them.
Oddly the one from archive.org was a relatively quick fix since 99% of the errors were from a paragraph style that incorrectly specified a width using a syntax that was never part of the various CSS versions. After that, the issues from a PDF to epub conversion were left (paragraphs split, images at random locations, etc.).
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