I've had the problem of Office files on my iMac not opening for some time now. I have a laptop with Office installed and an iMac, my laptop works fine (with pretty much identical OS and applications installed) but my iMac couldn't open files. I tried all
the options outlined above and had some initial levels of success, sometimes a file would open, but next time document tried to load, the same issue. I was up to date on OS and Office.
So in the end I bit the bullet. I wiped my HD and re-installed El Capitan and all my applications (including Suitcase).
Now Word and Excel have opened up all my documents fine, no issues.
My personal opinion is that somewhere along the line, with multiple installs and updates of OS and Office, something was clashing with the two.
This, albeit drastic and time consuming, course of action seems to have worked, fingers crossed it lasts!