Hi whatisk,
It's probably quite safe to ignore those messages and buttons. They fix very specific problems. If your EPUBs work on your reading device, then just ignore them.
However, to explain a little further, "Cover Info Thumbnail Missing" refers to a problem encountered with iBook (I think); the "Cover image uses SVG" button was added to strip the SVG from the cover image xhtml page because covers were not displaying correctly on my Sony Reader; and "Cover image not prioritised" is useful if you have CDisplayEx installed and have added EPUB to the list of files CDisplayEx creates previews of in Windows Explorer, but since it only displays the first image it finds, this button copies the cover file to a filename that will almost certainly be the first found (and resizes it to thumbnail size if you choose that option)! (For this last one, there is more information available at
https://wiki.mobileread.com/wiki/Thumbnails and
https://www.mobileread.com/forums/showthread.php?t=35505)