Is Quora corrupt /w single users with multiple accounts who abuse the petition system to ban people?

I'm not part of quora's community, but if there's one thing true of humanity, it's that absolute power corrupts absolutely.
If power, in the form of moderation votes, is granted automatically, then there shall be people, and especially those who are most motivated and idealistic, who seek it in order to reshape the site in the form they see fit. They do not have to be a majority, merely more unified than the rest, but ultimately the extent of influence is nigh unlimited, as others will opt to stand aside: either accepting the situation (if cognizant of it) or leaving rather than attempting to fight back, which as you've observed is futile.
Again, this is not unique to quora. Have you seen the film dirty harry? Reality would not go down as it did. For the sake of peace, not justice, his colleagues would testify against any foul play. Judges would very likely play along. Only what is provable matters in formality. Facebook's various scandals go without saying. Nowadays, public opinion has become a force to be reckoned with, yet it is both rousted and distorted with ease through the correct pressure points.

You as an individual have a great deal of choice, a sliding scale of how much you decide to get involved. However, prithee mark my words - corruption cannot be uprooted, only masked, so long as there is power to sustain it. Don't let it get the best of you.