While I agree with several of your conclusions here, and you bring up very good points about there being no limiting factor on frivolous negative ratings, I think your solution is lacking and is only adding more complication to an already over complicated system. The system needs to be simplified, not added to. My suggestion is to have a standard of evidence of theft, violation of contractual agreement, or violation of applicable laws before leaving a negative rating. This solves the primary issue of there being no limiting factor to false or frivolous negative ratings with minimal effort, creates a simpler system, and less counter productive side effects.
standard of evidence of theft, violation of contractual agreement, or violation of applicable laws just means that people need to be smart enough to not leave certain bits of evidence behind while doing what they are doing. Not being able to leave someone feedback on a technicality doesn't seem fantastic either. In addition, untrustworthy behavior is incredibly subjective, so who will decide what warrants negative feedback?
If someone leaves a frivolous negative because they believe you are untrustworthy on the grounds that you bought your account for example. A fair number of people might find that significant and would rather it not be covered up because it isn't breaking any laws, agreements, and no theft was involved.