How can you refer to content ID from underlying URL while adding a new node?
It seems that the node in the original link is only for the source page, not for the page in question when Drupal determines which data to pass into the contextual filter. Instead, I wound up creating a new URL path pattern and filtering on the basis of that structure. Then I could add token-using links to various pages, with the contextual filter picking up the pattern of those paths.

You can find a full write-up at the Drupal forums:

https://www.drupal.org/node/2905210#comment-12238299

Thank you again for your help! :)