(1) I feel better that you are aware of the tagging issue and I'll stay out of it.  (2) Having the discussion in 4 places is inefficient , but I don't know of a solution.  (3) I don't think that a solution for controlling content can be "legislated" since there is no real method for enforcement, short of the methods used for enforcing BLP, which is itself problematic.  Theoretically, guidelines only document actual practices at WP, but since there is no consistency in practice, it follows that there could be no consistent guideline written.  I think that it needs to be left to the editors at the individual pages to determine the content of plot summaries.  Good luck, and I'm happy to chat about this more if you'd like.