Okay,
– I see the guide on Bidirectional Relationships is a filter that does exactly that… when you relate a post on one side, the code updates to make the inverse link on the other.
– @hube2’s alternate code is a variant that does not have the above code’s requirement of using the same field name on each side.
This clears up my Post-to-Post understanding.
But it doesn’t quite get me to finally determining which route is best for my “Event” type… Post or Taxonomy?