I think the easiest thing would be to install openoffice.org on your computer. It's free. It runs on Windows, Mac, or Linux. It reads MS Office .doc files, and with suitable filters, e.g. writer2xhtml and writer2epub, it can output pretty clean .xhtml or a fairly decently formatted epub itself.
For info on the "writer2epub" extension, see this thread:
https://www.mobileread.com/forums/showthread.php?t=69636
Also, if you happen to be running Windows, look into Atlantis Word Processor. It is inexpensive, and also can export epubs directly. You may have to convert your .doc files to .rtf format in Word 2000, though. I don't recall whether it can read word .doc directly.
Information on Atlantis can be found in this thread:
https://www.mobileread.com/forums/showthread.php?t=48775
Either of these options would have had you producing an epubcheck-compliant epub in less time than you have already spent trying these workarounds.
Some tweaking in Sigil might still be necessary, though.
Albert