I like your ideas and I agree especially to these statements:

If rewards are needed, they will definitely fail again.
Witnesses have witness rewards. Managing abusing and spam would be great if witnesses did.

If 70% or more of the witnesses agree, Steemit can start Downvote.

This sounds similar to the idea I recently stated. As the job of a top witness also involves protecting the blockchain, I think it would be a very good solution to handle clear cases of abuse in this way.