So, I don't understand this:
Checking the SharePoint list shows a blank record with entitled "NEW", but no other records inserted accordingly.
What does that mean? Are none of your form values getting submitted to the list?
As to this:
Running the form a second time and then selecting the newly created ID so as to update it with new or even blank records generates the error message
I must not have explained well what I would like you to do - I don't want you to run the form a second time. I want you to open the form one time. Submit your list item. Leave the form open. Make sure in the form, in the ShPId field (or whatever you've used for your /id param) gets returned by the command - that is, make sure you have an ID number. Don't close the form. Change something in your form. Submit again and see if you have an error or an update.
However, it sounds as if you have another issue if you are just getting a blank record created - like your mapping may not be correct?