URL specific form for anonymous user I'd like to investigate if the following scenario is plausible somehow in Drupal 7:

The creation of a public form (node form/webform or whatever) with many instances so that each instance can be accessed only by a specific URL and can be filled by an anonymous user. The anonymous user should be able to fill in some of the fields, save his progress and come back later to fill some more using the same URL.

This scenario would allow the admin to create new instances, pre-fill some fields and send the form url by email to the final anonymous user.

The reason for all this is basically to avoid user registration and hold each user's submission to his unique URL form.
Any instance of a Webform is a node so each anonymous user get his own submissionreference. So e.g the name of the form is harrypotter the user is getting harrypotter/1 
Where 1 is the submission nr.

In rules you are able to get the submission id, get the content and redirect to the form again.

However you have to protect it again other users. Therefore you can use another Web form in front of it which ask for a pincode. You can redirect the user with the help of the pincode and rules to the correct submissionid