URL specific form for anonymous user
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