We would like to request an enhancement to be able to only click "Save & Close" once to complete the case record.

Issue reported and resolution provided from MSFT Tech:

The NavigateToMainForm function adds the target form to the browser's History stack.

For example if you're on the Contacts form and you perform NavigateToMainForm("Accounts"), clicking on your browser's back button will take you from the Accounts form back to the Contacts form. The "Close" part of the Save & Close button is the same thing as pressing the browser's back button: it returns you to the record you came from.

What is happening in your case that you're on the Case form, and you're performing a NavigateToMainForm to that exact same record. So clicking "Save & Close" saves any changes you've made and returns you to the record you came from which happens to be that same one, that's why you will need to click twice in the button so it returns to the All Cases view or the default view you have configured.