Afaik there is no way. There is a list at http://<site>/NintexForms/forms/allitems.aspx that stores your forms, but if you delete the form from the list it is deleted from this library as well and I could not find that deleted form in the recycle bin (neither in the admin recycle bin).
Sadly no. If you've saved the list as a template before the form is included in that template. If not then I think it's gone forever.
After deleting the form, there is in fact still an .XML file in the NintexForms library, but unable to Import this to the list. We noticed that when we published the default template to the list where the form was deleted that the Form List ID changes, but not sure where we can modify the .XML to change this ID back.
We ended up having to take a SQL Server backup from a previous date (night before), rename and attach this DB backup as a new database and point the affected site to this new DB instance and restore the site.
Are you sure the XML you found is the form you deleted? I'd rather think its just some other form.
However, glad you could restore your form. Please mark the answer as correct which is most appropriate for you so other users can see that there has been an answer found for your question.
Thanks
Cheers
Philipp
It would be good if Nintex Support could provide an option to restore deleted forms.