calibre does not correct the typical errors the checkers complain about, only errors that are likely to prevent your EPUB from actually working. And yes, you can unzip an EPUB correct errors and tell calibre to convert the OPF file to EPUB. See for
http://calibre.kovidgoyal.net/wiki/ADEQuirks for a subset of "errors" calibre will correct.
And almost all attribute not allowed errors can be safely ignored, with a few exceptions.