Commenting option for each paragraph of a node body
Have you looked at modules that may provide similar functionality like:


https://drupal.org/project/annotator
https://drupal.org/project/annotate


Other links:


https://groups.drupal.org/node/72748
https://github.com/patrickmj/Open-Review
https://groups.drupal.org/node/10875


They may not solve the problem you have but they may have code solutions that can help you go down the correct track