Solved! Adding a new referenced node

The solution to today’s problem was the Node Reference URL Widget. It does exactly what I want: I was able to add a link to the bottom of a convict index node to create an ‘additional information’, automatically linked to the index node.

When the ‘additional information’ node is created, the reference to the index node and the back-reference from the index node to the additional information node are both visible.

I was confused at first because I was trying to add the node reference field from the index content type. To create the link in the right direction, I had to add the node reference field to the additional information node instead. Here’s the diagram of the relationship between the two content types, managed by the awesome Node Relationships module:

Thanks, quicksketch and markus_petrux!