Here is my thinking. We have our incentives misaligned. steemit has more valuable content than spam or plagiarism. Minnows barely earn any curation and for whales loss of curation doesn't matter much.
So what if make flags cost 3% or even 4% voting power instead of 2%. There is less incentive to flag. Positive reinforcement works better than negative reinforcement. There will always be abusers and bad actors.
Not many would go around flagging left and right if they had to choose between 3 upvotes+positive interaction+curation rewards Vs 2 Flags+negativity.
People will require of themselves a very good reason to use their VP on flags. So naturally the flags would concentrate on content than can easily be identified as "Bad for steemit" (Spam, plagiarized content, promoting terrorism, spreading false info etc.)
Don't try to control actions. Instead, adjust the incentives and actions would follow.
Don't try to make things perfect. Keep it simple. Keep it elegant. Too many moving parts means too much complexity means too many ways things could go south.
I agree with your underlying analysis of incentives!
I think charging more VP for a flag could be a very elegant solution, to ease things in the right direction.