First, you’d be better off doing this work in form_valid rather than get_success_url.
You should also have your clean function for “when” in the form and not perform any additional validation in the view. The form_valid method should only be called once the entire form has been verified correct.
You can call super in your form_valid method and save the return value in a variable, update the objects, then return that saved value from your form_valid method.
See the docs and examples at Many-to-many relationships | Django documentation | Django for working with ManyToManyFields.