Interesting approach. I have a similar need. I was going to hide the sections of them form that we don't want people to edit after it has been submitted except by certain groups. I was then going to show a note on the form stating "Certain sections of this form have been hidden to prevent editing after the request has been submitted. Open the form in display mode to view these sections." I like your idea, but there are some fields we want to allow to be edited and that would prevent them from doing so.

It would be nice if Nintex had a feature so you could create a rule to prevent editing of all fields within a group. That would make scenarios such as yours much easier especially when the form has lots of fields.

(I am adding my own workaround and mark it as the solution because there does not seem to be any other solution)

I added a dummy, read-only field to the form and created a rule to set it as Required when the user is not a member of a certain SharePoint group.  This causes the user to be unable to save the form using the Submit button.  I also placed a label explaining that the user should use the Close button to prevent changes to the form.

In this method, the read-only user can still hit the Submit button, but it will not do anything.  It will point the user to the required field, but that field is disabled, so the user will have no other option but to select Cancel.