how to access webform submission data from a custom content type
One way to accomplish that would be to use the https://www.drupal.org/project/webform_entity_handler module. It is not very widely used module yet but I've used it, it was simple to use and worked well. What that module does it allows you to create arbitrary entity from a webform submission (like the results node). So you could store the input, submission id and whatever else you need to this entity at this time.
Then feeds can update this same entity with the results if you know unique identifier like the submission id.