Yeah... not sure how much you have dealt with getting people to remove their votes but its not easy. There is no good way to contact people, some people log out for days/weeks at a time, some people just don't care at all.
Sometimes you don't discover a post is plagiarized until the last minute, say 2 hours until payout, and there is no way to get 10 different people to remove votes in that time.
There is no practical way to do it without flagging.
Maybe, but it's still an open question as to whether the harm of flag wars outweighs the harm of some percentage of low quality posts earning unjustified rewards.
I'd be ok with getting rid of flagging if there were some other mechanism to deal with plagiarism/abuse, but there isn't an alternative.
I think that being able to remove the financial incentive for plagiarism helps to somewhat keep it down. If you make it so that there is no punishment whatsoever I think people would be encouraged to do it even more than they do now.