I did not read all the posts, however was a new chain installed with the original cassette and tested for skipping?
Had a customer back in 1999 that bought a new Rockhopper from us to commute with. He used the bottom 3 gears. Period, never shifted to lower gears. Within 500 miles he wore them out, hooked and ugly. Smallest cogs wear the quickest, and if used the majority of the time, it will show, but I never take a cog checker or mechanic's eye for fact, I throw a new chain on and try to get those little guys to skip. If they don't then why are they judged as worn out?