Actually, I think even if a majority of users don't vote in the way that was intended from the beginning you might still be able to handle it because you can th

Actually, I think even if a majority of users don't vote in the way that was intended from the beginning you might still be able to handle it because you can throw away users whose "quality" and "agreement" votes are most correlated, which I think is the most likely way users would deviate from the intended voting system. Now, if you had a majority of users who all voted the same but were not correlated with "agreement" (e.x. "vote according to the day of the week" or something), or if only very few users were voting correctly, then it might be difficult to distinguish, but that seems unlikely.