And I'd like to display the version in a notification in the workflow too, and the reference field I found in Item Properties always returns 1.0 which is obviously not correct.
The Item Properties - Version reference will display the version number of the Item and not the form.
Because the Form itself is not saved or appended to the actual item you are editing, or new item that doesn't exist yet, or added to the List itself, but in the Nintex content database you will not be able to pull the current version number. That information is stored in the database. You could use the Enterprise SQL control and pull the information from the database, but I do not recommend that you query the nintex database directly just as you wouldn't the sharepoint database.
I do not know of a REST or webservice call that you could use to get this information either, but that may exist somewhere.
I did find that the version number showing in the Version popup when you click the button in the ribbon goes to
Which means it is complied by an application page that is looking up this information.
It doesn't look like you can reference the form version number directly without an extra activity.