Here are some ideas worth exploring:
You have a text field (token value) that is hidden + disabled if the article id of the form is not set (ie. first edit)
A text field (confirmed) is set to CONFIRMED=0
The form is entered (first time) goes to thank you page saying to retrieve the token which is generated and stored.
The email link contains a random id and a link
The link is clicked and the form goes to edit mode.
The token field displays - Other fields can be hidden based on the value of confirmed or other value
The submit is entered which then validates the token field
If successful the form is saved and text field (confirmed) is set to 1